Absurdly Sharp Blade: Affably Evil. Ty Lee is a very pleasant person who acts respectful towards her friends and does not hold grudges but she assists Azula in her plans to capture the avatar
Sokka's Space Sword. The sharpness of the sword actually works against Sokka in the finale. When he tries to slow himself and Toph down by stabbing into the hull of an airship, it shears through the metal until they reach the bottom and keep falling.
To a lesser extent, Zuko's sabres. Zuko uses them to deflect rocks without them suffering so much as a dent.
Also Mai's throwing daggers, being capable of nailing people to solid metal just by throwing them.
Waterbenders can use water to cut through metal, sort of like a water jet cutter. In the first season finale, Aang slices apart the metal superstructures of Fire Nation ships using this.
FireLord Ozai. That scar on his son Zuko's face and his daughter Azula's heartless, sadistic nature are both his doing.
Toph's parents are mostly just neglectful, sheltering and coddling their girl instead of respecting her earthbending skills, but sending bounty hunters after her when she runs off (and assuming she must have been kidnapped by the Avatar) crosses over into stupidity.
Mai's parents--though not as bad as Zuko's or Toph's cases, it's revealed that she couldn't do much of anything except sit still and be quiet. If she made a comment at a dinner party, she got in trouble, if she fidgeted, she got in trouble. Heck, if she hugged her dad in public, she probably got in trouble. All because her parents just wanted to get higher and higher on the social ladder...and then they pretty much put her aside when her little brother Tom-Tom was born.
Action Girl: More often than not, any A and B character who has two X chromosomes is an action girl.
The 'four elements' wizard in the first episode of Book 2: Earth is based directly on Akiro, Mako's early role in the movie Conan the Barbarian.
When Piandao tells Sokka to think of the sword as "a part of your own body", it may come to the mind of certain viewers that his voice actor, Robert Patrick, has also played a character in Terminator who actually has swords as a part of his body.
In the episode Ember Island Players, Actor Zuko is voiced by the older brother of Dante Basco, the voice of the real Zuko.
Aerith and Bob: The names for most cultures seem to be Chinese and Japanese mixed together, with a few other things thrown in (Zuko, and Iroh look like Japanese names, Zhao and Ozai are Chinese; they're all from the Fire Nation). This trope happens if you're Chinese or Japanese - names that sound normal to you mixed with names that really don't. There are also a few names that seem "normal" to Western audiences, like Lee, Mai and Jet.
Mai is a Vietnamese name, quite possibly the only one in the series. Song is a Chinese surname.
Actually the 3 guys from the Foggy Swamp Tribe also have Vietnamese names, and presumably the rest of the tribe do too.
There's also a few that don't fit in anywhere - most notably Toph, Azula secondarily (looks Japanese but couldn't be since Japanese doesn't have /l/).
According to the characters written in certain episodes, Toph is an alternate spelling/pronunciation for either of the Chinese names Tuòfú or Tuofu. No word on whether that was the intention from the beginning or if it was just trying to find characters that fit. The Other Wiki says that Toph's name is actually means 'supported Lotus' in Mandarin. Her mother's name is Poppy however.
Also, if you use the r/l switch, as sometimes happens, you get a Meaningful Name for the girl that slings blue fire around.
Already meaningful, as Azula's name was to be "Zula" (to mirror Zuko), but was changed to Azula to capture the Spanish word Azul (blue), which is equally left-field in a world that is obviously South and East Asian-themed.
It would seem that Azula was named after her grandfather Azulon, who is named after Azulong, or The Azure Dragon of the East, a Chinese constellation symbol.
Double points for Zuko, whose name is not only both a Japanese and Chinese name, but is also a Meaningful Name in both languages. In Chinese, the name Zuko means "Conceited, proud of one's self", which seemed to fit his character rather nicely for a good portion of Season 1. In Japanese, the name 'Zuko' also means "sound of swishing swords", clearly in reference to both his swords and skilled swordsmanship.
Zuko is also fairly close to the Filipino word "Suko", which can mean "madness" or "angry" in one dialect, or "surrender" in another. While the second meaning doesn't fit him, the first meaning definitely fits him, especially in the first season.
In the episode "The Serpent's Pass" the travelers who are being escorted through the serpents pass decide to name their child born just after they reach the other side Hope.
One of the partygoers in "The Beach" is named "Ruon-Jian". It's pronounced "Ron Jon".
An Aesop: Lots of them. Many, many episodes have morals to impart, such as "The Kyoshi Warriors" ("sexism is bad"), "The Fortuneteller" ("we make our own destiny") and "The Deserter" ("impatience is bad"). Few of them are broken, lost, or otherwise faulty—or so Anvilicious as to feel like being on the receiving end of a sermon.
All Monks Know Kung Fu: Every single one of the Air Nomads knew airbending due to their culture's high level of spirituality. Also, the Fire Sages are all master firebenders.
All There in the Manual: The main purpose of the Nickelodeon site is to provide supplementary information while not using up valuable air time.
The Avatar comics which appeared in various Nickelodeon publications throughout the run of the series also fill in some important details, particularly the ones set between Seasons 2 and 3, which reveal what happened to Ba Sing Se after Azula conquered it, and how Zuko and Mai got together.
Occasionally subverted throughout the series, and then fully subverted as the third season goes on and we meet the citizens of the Fire Nation; Aang even lampshades the subversion at the end of "The Avatar and the Fire Lord": "Everyone is capable of great good and great evil." And then there's Hama, the southern Waterbender who subverts Always Lawful Good.
Always Save the Girl: Aang decides saving Katara is more important than finishing opening the chakras for the Avatar state, which was something that Iroh fully agreed with.
Ambiguously Bi: Ty Lee shows unusual interest in Azula, which goes almost to the point of obsession. On Ember Island, several boys took interest in Ty Lee on the beach, to which she was completely oblivious and preferred the company of Azula. She also had a brief interest she took in Sokka.
Anachronism Stew: In "The Avatar and the Firelord", Firelord Sozin's portrait has several soldiers near the bottom, wearing uniforms that supposedly didn't come around until about a century after the portrait would have been painted. The soldiers should be wearing the uniforms the Northern Water Tribe had at the end of Season 1.
Those were Fire Navy uniforms that the Northern water tribe had, the Fire Army presumably had different uniforms.
The Avatar Extras get into it too. At the end of "The Chase", when the Gaang plus Zuko all attack Azula, it says, "This is the first time we've seen all four elements attacking at once." A beat later, it adds, "...Plus Sokka."
Angrish: Sokka's reaction towards Toph waking him up in "Bitter Work." After considering the sleepless marathon he had to go through in the previous episode, his frustration is well justified here. The Avatar Extras note that the writers did have actual dialog for Sokka, but the voice actor improvised the grumbled angrish instead.
Angst Dissonance: An in-universe example; while Sokka and Zuko are heading towards the Boiling Rock and bonding, Sokka says, "My first girlfriend turned into the moon." After a pause, the only thing Zuko can say is, "That's rough, buddy." What else can he say to something like that?
Angst Coma: In Season 2, Zuko undergoes a severe fever and enters a coma in which he has vivid dreams in which his uncle and sister appear as dragons and argue over his life choices. Iroh says that this is "not a natural illness" and the whole thing is apparently caused solely by Zuko's premature Heel Face Turn.
Katara in "The Puppetmaster" when Hama manipulates Sokka and Aang into attacking her.
Arc Words: "I must regain my honor." There are story sequences built around this phrase for Zuko, Aang, and Sokka ("the Boiling Rock")
"Destiny" is another major theme, seeing as the show is primarily influenced by Eastern Philosophy. It ends up applying to every single character, and together their stories send the message that while you always have a destiny, you are the only one who can choose to follow it.
While not exactly a major theme, the word "crazy" is used repeatedly: It's associated with Azula several times ("...crazy blue firebending", "Girls are crazy!", "She's crazy and she needs to go down.") and is also used to dismiss ideas and people as unimportant. Sokka uses it a lot during "The Fortuneteller"; during "The Beach", Toph says: "Guys, you're all gonna think I'm crazy, but it feels like there's a metal man coming." By the end, Azula becomes the sort of person who's a shorthand for "unimportant" and "easily dismissed."
Which can lead to Unfortunate Implications, as some of this indicates a perhaps unconscious attitude that people with mental illnesses are dangerous, evil, and/or insignificant.
The first season is bookended by Iroh telling Zuko "A man needs his rest."
Lampshaded by Katara crossing her fingers and repeating "communityservice". Gotta love it.
The Earth King's first line in his titular episode also makes good use of this trope.
Earth King: You invade my palace, lay waste to all my guards, break down my fancy door, and you expect me to trust you?!
Extras Popup: And he loved that fancy door!
And do I even need add "the malicious destruction of cabbages?
Artistic License Astronomy: Sozin's "Comet" is something of a misnomer. Comets are seen for days or possibly weeks as they take their sweet time to fly around the sun, are made of much more ice and earth than fire, and rarely intersect the atmosphere and live to tell their tale again. There is a class of objects that do--Earth-grazing fireballs--but "Sozin's Earthgrazer" isn't anywhere near as poetic, and doesn't carry that ancient "comets are harbingers of doom" mystique. In addition, the distinction wasn't always as clear in our world, either, so a culture with no advanced astronomy could easily just call any flying space object comets, and the word "comet" comes from Ancient Greek, the language of a culture in which people also thought the world consisted of a total of four (guess which) elements.
Word of God owns up to the eclipse's visibility all over the world being impossible.
It is plausible if Omashu and the Fire Nation Capitol just happened to both be in the swath covered in total eclipse. Intercontinental total eclipse zones are possible in real life.
A similar issue appears in "The Siege of the North." Despite being located at the north pole, the sun rises and sets as it would below the arctic line. As mentioned in "The Boy in the Iceberg" by Sokka, midnight sun does in fact exist in the world of the Avatar. As such, for day and night to cycle during a normal twenty four hour period in the north pole is not plausible around the solstices. That said, it was not said how long after the winder solstice it was until the G Aang reached the North Pole.
It can be argued that, seeing as the Northern Water Tribe capital is on the coast, they could still be fairly far from the "true" North Pole.
Fire does not have a concussive effect unless it is part of an explosion, and even then it's the rapidly expanding gases (fire is a form of plasma) that cause the person or object to be pushed back.
Mai's daggers should not cause people to go flying back into walls unless she's somehow Gambit in disguise. Also note that at the speed she throws them, they should just cut right through the clothes instead of dragging people with them.
Though there are some uncommon aversions as well: Azula's fire is extra hot and therefore blue, and Aang makes a point to cut the surface tension of the water he falls into, also averting Soft Water.
Art Evolution: In a short series like this, and an animated one to boot, one normally doesn't find art evolution. However, it does occur; for instance, compare the size and shape of Katara's eyes (They shift from a noticeably Tibetan (or maybe Inuit) appearance to the more standard large anime eyes) in the first few episodes to the last few episodes, Sokka's entire facial structure and even Aang's face changes somewhat.
Also, in a more subtle example, as time goes on, Zuko's scar seems to become a much less dominant feature of his face, partially because he loses his Bald of Evil and lets his hair grow long enough that it covers a lot of the scar.
Art Shift: The flashbacks in Season 1, Episode 11, "The Great Divide".
Asleep for Days: Aang is knocked unconscious by Azula's lightning bolt so long that his hair had enough time to grow back.
Asshole Victim: All signs point to Fire Lord Azulon not being a very nice person. Word of God and background material show him as ruling the Fire Nation with an iron fist and carrying out the war against the other nations with brutality only matched by Ozai. Of particular note is the genocide carried out against the Southern Water Tribe's Waterbenders and the raids carried out against said tribes. (These raids eventually resulted in the death of Katara's mother.) Not to mention the fact that he was willing to have his innocent grandson killed just to prove a point to Ozai. Other than possibly Iroh, it's unlikely anyone really missed him when Ozai and Ursa killed him.
And quite a few badasses are really, really cute. (see Badass Adorable for more details)
No love for the titular character? seriously...yes, he might've had the advantage of being a Physical God, but think about it; a 12-year-old-martial-pacifist-warrior-monk-child went and ended a 100 year conflict after surviving the genocide of his people, and did it without killing his primary nemesis. That's pretty damn badass I'd say.
Practically required to be in the Order of the White Lotus.
Especially Bumi. Bumi does not escape from the army occupying his city and holding him captive; the army escapes from Bumi.
In addition to retaking his own city (and escaping captivity with only his face) Bumi used this to his advantage when he first met Aang after the 100 year time skip. Knowing Aang would try to outsmart him, Bumi put himself in a position where Aang can try to Loophole Abuse his way into fighting a frail old man instead of his two large and muscular guards. Aang learned the hard way why Bumi was king.
Badass Normal: Sokka, Hakoda, Suki, Ty Lee, Mai, Jet and his freedom fighters, Piandao.
The "Sokka's Master" episode may count as a lampshade When Sokka is dissatisfied with his being normal, the group suggests that he Take a Level In Badass.
Bad Bad Acting: The Ember Island Players, though in Sokka's actor case he actually takes Sokka's advice to heart and eventually does better. Also, when the Gaang tries to get Katara (a waterbender) arrested for earthbending so she can rescue the earthbenders that have already been arrested. Things would have been so much easier if they had Toph at the time, but sadly, they did not.
Badbutt: Toph, especially while training Aang, and Jet.
The Bad Guy Wins: Fire Lord Ozai may not have succeeded in his plans, but Fire Lord Sozin SURE did. Though he realized eventually that he should have listened to Roku. And of course, in the play written by the Fire Nation, Fire Lord Ozai succeeds in winning the war, even though strangely enough Zhao does not succeed in becoming the most famous man in the Fire Nation.
Aang is very attached to Appa, and Katara, and putting either in danger is a very good way to have him go Avatar-state crazy on your ass.
Sokka loses it when Azula taunts him with descriptions of his captured girlfriend. For context, he knew she was taunting everyone to get them to run out the clock and he still lost control. Granted, Azula's just that good at manipulation.
He ends up fighting with Hahn over Yue, who the warrior clearly doesn't appreciate.
When Aang accidentally burns Katara, Sokka tackles him to the ground. Nobody hurts his little sister.
Anything that reminds Katara of what happened to her mother is enough to send her flying out of control, to the point where she bloodbends a man whom she thought was the murderer. She eventually learns to rein in this part of herself.
Harm to Aang is her second berserk button.
Iroh rarely gets truly upset throughout the entire series. However, he has a deep connection with the spirits, which is why we see him none too pleased when he realizes Zhao's plan to defeat the Northern Water Tribe by killing the moon spirit.
Iroh: (to Zhao) Whatever you do to that spirit, I'll unleash on you tenfold! Let it go NOW!
Jet and absolutely anything having to do with the Fire Nation. His inability to let go of his intense Fantastic Racism is what inadvertantly leads to his death.
Berserker Tears: When Sokka discovers Azula captured Suki and the rest of the Kyoshi Warriors (including referring to Suki as "[her] favorite prisoner") during Day of Black Sun, Sokka gets these and flips his shit.
Biological Mashup: Almost all of the fauna in the Avatar world is this. So much so, that simple, non-mashup animals are considered weird and inexplicable.
Bishonen: Oddly enough, Fire LordOzai is one. Makes more sense when considering that, according to Bryke, he's supposed to look like an older, eviler un-scarred Zuko.
Bits of Me Keep Passing Out: Sokka during the fight with Ty Lee; she was paralyzing his limbs one by one, yet he still tried to fight. Another similar incident happened when he was paralyzed by June's beast, and just when he started to gain some control over his hand, a pile of blocks fell on top of him.
Black and White Morality: Played straight at the extremes: Aang and company are good, Ozai and Azula are bad. Subverted with prejudice practically everywhere else, as a wide variety of sympathetic Fire Nation characters are introduced, while both the Earth Kingdom and the Water Tribe each get at least one villainous figure. One of Aang's most important lessons is that good and evil can come from anywhere.
Bluff the Eavesdropper: Azula has Mai and Ty Lee purposely reveal that they're Fire Nation in disguise... to the pair of Dai Li agents they knew were eavesdropping, overhead. She knows they'll ferry the news back to Long Feng, and that he won't be able to resist the opportunity to use it against her. So she isn't surprised when those same agents bring her to his cell, where he coerces her into helping him under threat of exposing her to the Earth King. Azula "reluctantly" agrees. It doesn't end well for Long Feng.
Subverted in that we eventually find out how the fight turned out.
A very subtle example may occur in "Lake Laogai". With Jet mortally wounded, his comrades Longshot and Smellerbee tell Aang's group to go on ahead. This is followed by a shot of Longshot notching an arrow in anticipation of incoming enemies. The trio are never seen again, not even when nearly every living good guy teams up near the end of Book 3.
This one was also subverted as of the sequel comic "The Promise", where Smellerbee and Longshot make an appearance.
Bond Villain Stupidity: The Fire Nation in general. For a nation dedicated to wiping out everyone with Elemental Powers besides themselves, they seem remarkably intent on leaving the Benders from other nations sitting around in prisons rather than just executing them - and confirmed traitors like Iroh, as well.
It should be noted that imprisonment would be a more viable choice since execution of the benders may turn more people against them including the very citizens of the Fire Nation. From the public's point of view, killing an enemy in the battlefield is different from executing an already defeated and imprisoned foe.
It should also be noted that this is a kid's show that tends to take a Never Say "Die" attitude with respect to named characters. Execution is a bit... adult.
It still can be justified in-universe: Keeping in mind than the War lasted one hundred years, killing each new generation of benders would establish the entire Fire Nation as unbelievably evil from the point of view of non-benders, that they can't subdue due to their sheer number (The Air Nomads being the exception, since they were a sufficiently small population to Kill'Em All at once). Simply neutralizing them work better from a P.R. stand-point.
Book Ends: A pillar of light shooting into the sky in the first and last episode.
Boring but Practical: Very commonly used throughout the show. Weapons and armor worn by the characters tend to be much closer to actual equipment used in history with very few unnecessary frills.
The Fire Nation Navy used to have shoulder spikes on their uniforms, but eliminated them soon during the war due the spikes constantly getting stuck into things.
The best part? This was the creator's answer to Nickelodeon's request for them to give Aang armor so they could make it into a toy.
Boobs of Steel: Ty Lee has a rather sizable chest, the largest in Azula's group, and is easily the physically strongest and most agile character in the series who isn't using bending, male or female (her name even means "extreme strength").
Also the leader of the Gan Jin tribe from The Divide. She's a fairly buxom lady and holds her own against the canyon crawlers without any bending.
Entirely averted with regards to bending. Neither Katara, Toph nor Azula are especially well endowed for their ages, but all are among the best in the world in their respective elements. Indeed, Toph may well be one of the greatest Earthbenders in history and she's as flat chested as one would expect a 12 year old girl to be.
Break the Cutie: Pretty much every character ever. Katara, Sokka, Aang, Zuko, and Jet all get flashback scenes showing them as cute, chubby-cheeked little kids that have horrible things happen to them. The rest of the Freedom Fighters, Ty Lee, and Mai are implied to have gone through this - all of the Freedom Fighters are orphans who have been hurt by the war, Mai withdrew into herself as a result of her oppressive childhood, and Ty Lee snapped and ran away to the circus because she was treated like she had no personal identity. Even Azula could arguably fall under this - her deep desire for her parent's affection, likely based on her mother's unspoken preference for Zuko, is part of what makes her the monster she is during the show.
Don't forget Appa and Hama. Appa had an entire episode showing him being treated as a universal punching bag. It was horribly depressing.
Break the Haughty: Zuko over the course of most of the series, which leads to serious character development on his part; while Azula (who starts out even haughtier) gets it all crammed into a few episodes and doesn't take it as well.
In the Book 1 episode "The Deserter", Sokka tries some Fire Flakes, which are pretty hot, but it's made much funnier when we meet Mai a season later in "Return To Omashu" and she snacks down on them like they're nothing.
Sokka developed a taste for them by the time "Ember Island Players" rolled arond.
Zuko mentioning the tsungi horn in Book 1 seems like an offhanded comment at first, if you don't remember that it actually appeared in "The Waterbending Scroll"; later we see Aang playing one in "The Headband". Iroh himself plays one in the last episode (presumably it's the one they bought).
Broken Glass Penalty: In the Iroh segment of "Tales of Ba Sing Se", a few kids are playing with a ball, and a window is broken. Iroh appears and the following dialogue occurs:
Iroh: It is usually best to admit mistakes when they occur, and to seek to restore honor.
A large man appears inside the house
Large man: When I'm through with you kids, the window won't be the only thing that's broken!
Iroh: But not this time! Run!
Brotherhood of Funny Hats: In the comic book story "No Benders Allowed" (printed in The Lost Adventures), Sokka starts a club like this along with the Gaang's non-bender allies. They eventually let the benders into the club, provided they "pledge allegiance to my bendless brethren, and admit that no bending can equal the might of the noble boomerang".
Bubble Pipe: Sokka when he's doing his Sherlock Holmes impression.
Bullet Time: Ample examples throughout the show, with Aang having the most.
Prominent in Toph's first episode; time freezes many times, showing how she feels the vibrations in the ground to "see" what's going on. In that same episode, a rock flies by her face in slo-mo.
Bullying a Dragon: When Aang encounters Zhao in The Deserter, the whole battle is Aang antagonizing Zhao so the latter will try to blast the former with fire. Zhao didn't realize he had been blasting his own boats until Aang pointed it out.
The Bus Came Back: This show is notable for bringing characters who appeared at first to be one-shot extras back for reappearances (often spoiling the surprise with a Spoiler Recap at the beginning of the episodes in which they reappear).
Most notably, in "Day of Black Sun", characters from "Jet", "The Northern Air Temple", "The Swamp", and "The Blind Bandit" show up… as well as Hakoda, Bato, and the warriors of the Southern Water Tribe. A number of characters in the finale, too (see Back for the Finale, above).
Jet, Longshot, and Smellerbee in Season 2.
Also, Suki at various points: "The Serpent's Pass", "Appa's Lost Days", then becoming a Guest Star Party Member from "The Boiling Rock" until the end of the show.
On the villain side, the pirates from "The Waterbending Scroll" showed up again in "The Waterbending Master".
June the Bounty Hunter, first appearing in Season 1's "Bato of the Water Tribe", and then reappearing in Season 3's "Sozin Comet, Part 1: The Phoenix King".
Butt Monkey: Sokka, on several occasions, particularly during the first half of the series. Sokka actually displays considerable growth over the course of the show, something few Butt Monkeys in other shows ever get to do.
By Wall That Is Holey: the Earth Kingdom's trademark disc projectiles with square holes in them (based on Chinese coins) turn out to be rather impractical, particularly in "The Avatar State".
Azula: Isn't it obvious? I'm about to celebrate becoming an only child!
Call a Smeerp a Rabbit: "Penguins". Played for laughs and subverted with the Earth King's pet bear; the crew goes on to name various animals that could be combined, but are left perplexed as it just says 'bear'. This trope is pulled throughout the entire universe of Avatar, which lends more humor to the situation.
Call to Agriculture: Subverted. The man who killed Katara's mother has a garden and it is implied that he spends quite some time on it, but he is still the cold and heartless man he was when he was an Admiral. It's just hard to tell because Katara is being really scary, and he's already a coward anyway.
During Season 2 and at the end of the Grand Finale, Uncle Iroh is "Called To Tea Service".
Calling Your Bathroom Breaks: The children that Sokka is trying to train in the first episode, just to illustrate how young they are and how hopeless Sokka's task is.
Can't Catch Up: Poor Sokka. Though he's The Smart Guy, occasionally gets a lucky hit in, and has a lot of non-combat skills, he's dead weight in any real fight because he doesn't have any elemental powers. Later, when he trains to become a swordsman, he's STILL outclassed. Even a skilled swordsman simply can't compete with people who can bend the elements to their whims... As a few have said, "Sokka is the Krillin of Avatar..."
Jet goes through this when trying to warn people that Zuko and Iroh are firebenders that are trying to infiltrate the city. After spending several days trying to get proof, he loses all credibility when he decides to suddenly attack them in front of a bunch of customers to get them to firebend in defense, which leads to his capture and Brainwashing by the Dai Li.
In Jet's first appearance, Sokka also went through this while trying to convince Aang and Katara that Jet planned on wiping out an entire village just to take out some Fire Nation soldiers stationed there.
Cat Smile: Jin, who owes a lot of her popularity to this.
Charles Atlas Superpower: Anyone with an extended lifespan is doing so through nonbending means; for instance, the Guru and Avatar Kyoshi both lived over 150 years, even though the Guru has no bending ability.
Charge Into Combat Cut: This trick is used a few times n the finale to cut between Aang's battle with Fire Lord Ozai and Zuko's battle with Azula.
Character Exaggeration: "The Ember Island Players" is an in-series example where just about everyone is absorbed by some single trait, often a comparatively minor one that they may not even have anymore.
Subverted with Haru in the episode "Imprisoned". He saves an old man with his illegal earthbending. Old man turns him in...
Chekhov's Gag: In the earliest episodes, Katara had a tendency to mess up her waterbending, and Sokka would get drenched. In "The Waterbending Master" Katara redirects a stream of water during a fight. Sokka gets blasted away by it, even though everyone else around him is just fine.
The warden's word that he would sooner jump into the boiling lake than tarnish his prisoners' escape record (then, zero). One episode later he is willing to have the gondola cable cut to prevent our heroes from escaping, even when it means his own demise.
Zuko's twin swords (seen on his wall early in the series), schematics for a drilling machine, a prototype war balloon, Katara's necklace twice (one used by Zuko to track the Gaang and once when Master Pakku realizes it belonged to Kanna, Katara's amulet of water from the North Pole oasis, the Lotus tile, Iroh's sandal, the scroll with the Lion Turtles at the Library and the bison whistle in particular since Sokka initially chided Aang for wasting money on it. One season later, it's still important.
The bison whistle is more of a Chekhov's Boomerang: It apparently serves its purpose at the end of the episode it first appears in, only to show up again halfway through the next season.
In the same episode, Zuko's ship stops in port so that Iroh can replace the White Lotus tile from his Pi Sho set.
The war balloon is a special one that deserves mention because it takes two seasons to resolve.
After Katara steals the waterbending scroll, she attempts to learn the water whip. Look at the diagram. It's the same waterbending move in the intro every week.
A minor one in "The Avatar State," Zuko and Iroh cut off their top-knots with a knife Zuko has on him. Guess what pops up six episodes later in "Zuko Alone?"
Made even more brilliant when you watch The Storm. Iroh redirects lightning during the titular storm. In season one. Mike and Bryan, I worship you.
Zuko using firebending to hold off hypothermia in "The Siege of the North" comes in handy when he gets locked in the cooler in "The Boiling Rock". (Which is arguably less fatal than freezing water, but still impressive.)
Katara's use of bloodbending in "The Southern Raiders".
Chekhov MIA: Koh the Face Stealer's parting words to Aang are "We'll meet again..." They don't, though given that Koh doesn't differentiate between individual Avatars, he probably meant eventually.
They do... just not in the televised bit of the series. There's a canon game that was online taking place between Books 2 and 3 where Aang goes to the spirit world to make sure that the Avatar line wasn't broken when Azula killed him. Koh both helps him find his former selves and acts as an impediment in his search.
Child Soldiers: Most of whom fit under the talented and tragic categories.
Katara: I haven't done this since I was a kid!
Aang: You still are a kid!
Chirping Crickets: Sometimes with a cough, but mostly with animal sounds - a duck in "The Fortuneteller", a defrosting frog in "The Blue Spirit" and a badger-toad in "The Western Air Temple", among others.
The Chosen One: Aang, although the reincarnation system makes it a bit muddled whether he's chosen or just following all his lives.
Katara was destined to be Aang's waterbending master. She always believed the Avatar would return and she released him from the iceberg. Gran Gran told her that their destinies "are now intertwined."
Toph was first seen as a vision and was therefore destined to be Aang's earthbending master.
Zuko (who, in keeping with the pattern of this series' use of this trope, is destined to be Aang's firebending master) just happened to be passing through the South Pole when Aang was discovered in the iceberg. He also found out that Avatar Roku was his great-grandfather and that his destiny was intertwined with that of the the Avatar. Aang even admits this outright.
Cincinnatus: Iroh turns down the offer of Fire Lord if his brother was defeated, since he knew Zuko would have to do it.
The clothing styles of Four Nations: Denizens of the Fire Nation wear red, gold, and black; The Water Tribes wear blue and white; the Air Nomads wear yellow and orange, and members of the Earth Kingdom wear greens and browns. The Air Nomads' colour is actually known as kavi, a variation of yellow worn by priests and monks. The writers did their research.
Also, Azula's blue fire, which indicates her cold, cruel detachment (It also shows her as a firebending prodigy since blue fire burns hotter) vs. Zuko's red (to indicate his heated emotional nature). Interestingly, the color combination was reversed for Aang and Ozai when they were energybending in the Finale.
There are exceptions to this rule:
In Book 2, Zuko and Iroh relocate to Ba Sing Se. In order to blend in they wear the customary clothing of the Earth Kingdom.
At the start of Book 3 the gaang captures a Fire Nation ship and pose as Fire Nation. In the second episode, the Gaang steals Fire Nation clothing in order to blend in.
In the final episode the Gaang is dressed in Earth Kingdom clothing. This symbolizes the integration of the four nations.
Jet and his gang wear clothing not matching any of the four nations. (However, this could actually still work with this trope by going along with the fact that while they live in the Earth Kingdom, they are really more of a law unto their own, signified by their patchwork, lost boys-style clothes.)
Ty Lee wears pink and at the end of the series is dressed in the customary uniform of the Kyoshi Warriors since she joined them despite being Fire Nation.
Pink is technically a shade of red and thus not really a subversion.
Though, arguably, these "exceptions" are actually just showing how the series conforms to this rule, since all people had to do to fake their nationality was change their clothes (and in Aang's case, hide his tattoos).
The color of benders' eyes correspond to the color of their element (green for earthbenders, blue for waterbenders, gold for firebenders, grey for airbenders) and to some degree, everyone's eyes tend to correspond to their nationality as well (which makes one wonder why no one suspected the blue/green/grey-eyed bunch of youngsters tramping around the Fire Nation of being the exact bunch of foreign youngsters they were expecting to try to overthrow the Fire Lord).
It could also be noted that, within the fire nation, eye colour does differ from person to person, often appearing to be coded by the characters alliance, for example, Zuko's eyes are light amber, signifying that, while he starts on the bad side, he is really good, Zhao's are dark orange, showing that he is ultimately corrupted, Azulas are so dark they are almost brown, and Iroh's are a nice pumpkiny yellow, showing his true alligence to the light.
Color-Coded Patrician: Princess Yue wears purple, when everyone else in the water tribes wear blue, including her father.
Comet of Doom: Sozin's Comet (even if it's not REALLY a comet), although the prediction of doom is hardly mystical; its arrival increases the Firebenders' power by a hundredfold. Fire Lord Sozin, for whom the comet was named, used it to launch a first strike against the Air Nomads, which completely eradicated them. His grandson, Fire Lord Ozai, attempted something similar in the finale.
Commuting on a Bus: In the third season, this happened to Uncle Iroh... for the first few episodes, he didn't even get any speaking parts! (Word of God is that this had already been planned that way and was not because of the sudden death of his prior voice actor, Mako.) Then he had exactly two speaking appearances in the entire rest of the season.
Companion Cube: Sokka's Boomerang and later, Space Earth Sword. Complete with "death".
Many, many examples. The umbrella from "The Fortuneteller" is found in Appa's luggage by sandbenders in "Appa's Lost Days". The eye-patch wearing Fire Nation commander from "Jet" shows up again in "The Cave of Two Lovers". The tsungi horn and ruby encrusted monkey statue Iroh buys in "The Waterbending Scroll" appear several times. War Minister Qin turns up at least once a season, usually prior to or during the unveiling of a new Fire Nation war machine. Sokka's attempt to reveal himself to Suki with a kiss in "The Boiling Rock" is a callback to when she did the same to him in "The Serpent's Pass". And so on.
Katara mentions in "Bitter Work" that the reason Aang is having so much trouble learning Earthbending is because Earth is the natural opposite of his element (Air). Later, in "The Avatar and The Firelord", Roku mentions conversationally that out of all the four elements, Water (the natural opposite to his element, Fire) was the one which was hardest for him to master.
The married couple first appears in Zuko Alone before appearing in the Serpent's Pass.
Iroh's journey to the spirit world was first referenced in the Winter Solstice Part 1 when he sees Spirit Aang. It is not actually mentioned until the Siege of the North.
The Badger Moles make their first appearance in the Cave of Two Lovers. Toph later mentions how she learned earthbending from the Badger Moles.
Additionally, the Badger Moles are seen briefly in season 2, episode 6 when Aang first decides that Toph should be his earth bending master. The moles are fixing the arena after the previous battles.
Characters often seen for brief moments make larger appearances in later episodes.
While Zuko and Iroh are hiding in Ba Sing Se, Zuko takes on the name Lee. While this is just a random name at that point, a season later we learn from Piandao that "There's a million Lees", so Zuko was essentially going by his world's version of "John Smith".
In The Boiling Rock, Zuko spends some time locked in a chilled prison cell, designed specifically to punish Firebenders, as part of a Batman Gambit by him and Sokka. He used the same chi technique that he learned at the end of the first season to stay warm, as demonstrated by his exhaled puff of flame when he is retrieved.
A particularly egregious example can be found in "The Awakening": Aang is standing among streams of lava, some of which flows literally between his feet. He sticks his wooden glider to the ground and leaves, and the glider immediately catches a fire and burns, but somehow this terrible heat doesn't bother Aang (or the other members of his gang) at all. So convection clearly exists, but it doesn't appear to affect humans.
Conveniently Interrupted Document: Zhao burned away references to The Day Of Black Sun, and indeed, any references to the Fire Nation at all in Wan Shi Ton's library (which served as the last straw for the spirit, and caused him to become hostile to humans).
Cool Tank: The Fire Nation makes wide use of Steampunk tanks with Firebenders as their weapons, but the coolest are the Earth Kingdom's tanks, which are incredibly tough worm-like contraptions powered entirely by Earthbending. As such, they can scale cliffs and staircases, attack conventionally with Earthbent rocks, or Earthbend their hulls to crush enemy tanks.
Could Have Been Messy: Many, many cases, on both sides. Aang will not kill people. He will, however, dump an avalanche of snow on top of masses of enemy soldiers while they're walking across a sheer mountain pathway seemingly very high up, sending them hurtling off. Just so long as the aftermath is unseen we can safely assume no one got hurt. Right?
Cross-Dressing Voices: Not used in the show itself (12 year old Aang is voiced by Zach Tyler Eisen), but Lampshaded in the play in The Ember Island Players, where Aang is played by a (visibly adult) woman (voiced by Rachel Dratch).
This doubles as a reference to Peter Pan, where the eponymous eternal child is traditionally played by a petite woman rather than a prepubescent boy.
Cruel Mercy: To both Ozai and Katara's mother's killer.
The Cuckoolander Was Right: In the Siege of the North Part 1, when talking about getting the Ocean and Moon Spirits' support, Aang says that, "Maybe they'll unleash a crazy amazing spirit attack on the Fire Nation." Both Yue and Katara give him weird looks. Guess what happensnext episode?
Culture Chop Suey: Sokka carries a boomerang, despite his water tribe culture being based off of the Inuit people.
The entire series is a mixture of cultures including but not limited to, Chinese, Japanese, Tibetan, and Inuit culture. The notion of the Avatar comes from India and the Hindu religion. There is also quite a bit of American Influence, as the show, following the style of Japanese Anime, is an American Cartoon.
Culture Clash: Zuko's sensibilities don't entirely align with Aang's even after he joins Team Avatar. For instance, in order to make up for having wronged Katara in the past, Zuko offers to help her murder an old man (who killed her mother, but still). Aang himself is subject to this as his upbringing taught him that killing was absolutely taboo, meanwhile his friends, who've all grown up in a world that's only known a century of war, have no problem with killing in battle.
Less Curbstompy, but still pretty much one sided, Pakku vs Katara shortly previous. Justified since he's an Old Master while she's a largely self-taught 14 year old.
Cursed with Awesome: Aang never wanted to be the Avatar, and ran away when the monks tried to separate him from Gyatso.
Cut Apart: In "Lake Laogai", Team Avatar opens the door to where they think Appa is, and Appa looks up at the opening door to see... Zuko. A mild Lampshading follows as Zuko snarks, "Expecting someone else?"
Also happens in "Imprisoned": Fire Nation troops walk around menacingly as the Gaang sleeps, the troops then knock on a door... which is opened by Haru.
Cute Bruiser: Toph. While not physically strong, she has a Boisterous Bruiser personality, shatters boulders with casual ease and is usually the go-to team member for sheer brute force.
Dance Line: in "The Headband", Aang is trying to teach some Fire Nation kids how to relax and have fun and does this at his secret dance party.
Dangerously Genre Savvy: Azula is very cunning, and has no reservations about resorting to the most dirty, underhanded tactics available if it will help her win. Case in point: in the second season finale, she sneaks up behind Aang while everyone else is busy staring in awe at his Transformation Sequence and sends a bolt of lightning into his back. After Zuko blatantly lies to her, saying that there's no way he could have survived, she sees through the obvious lie... and is summarily proven right. And then she turned Zuko's lie back against him.
The Avatars are supposed to be told about their identity on their sixteenth. Aang didn't get that luxury.
Sixteen is also the age in which Northern Water Tribe girls become eligible for marriage, much to poor Yue's dismay. It is unclear whether the same custom carries on to Katara's tribe. Though it seems unlikely since Kanna (Gran-Gran) ran away to the Southern Tribe specifically to avoid her arranged marriage.
Dark Action Girl: Azula and Mai. Ty Lee sort of counts, in that she's a bad guy and an action girl, but her personality is anything but dark.
Darker and Edgier: The series as a whole is much darker and more serious then one would think for a daytime Nickelodeon show. There is direct mention of people having died before the series, a handful of people who die during the series, multiple mentions of mass genocide (strictly along racial lines). There are also rather mature themes throughout, such as parental abuse, murder, scandals, and having one of the main protagonists nearly kill someone of their own free will.
Dawson Casting: aversion Aang and Toph are voiced by actors of about the correct age. The actors who play Katara and Sokka are not much older than their characters, either.
Played straight in Chile with Aang, whose male voice actor (René Pinochet) was almost 30! Justified that he was born prematurely and his voice hadn't changed much since childhood.
Deadpan Snarker: Several characters fit this one, including Sokka, Toph, Iroh, Azula and of course Mai.
Even Zuko gets in on it at times:
Zuko: Yeah, and then you can show [the Fire Lord] his baby pictures and all those happy memories will make him good again.
Aang: Do you really think that will work?
Deadly Dodging: several examples, like Aang vs Zhao in "The Deserter", but the one taking the cake would be that scholar being attacked by the platypus-bear in the beginning of "The Fortuneteller". Aunt Wu wasn't that far off.
Death Faked for You: Iroh claimed to have killed the last dragon, but he instead found them and didn't tell anyone, so they could live in peace.
Declarative Finger: Zuko had one when he was impersonating his Uncle giving him sage advice.
Zuko: How am I supposed to convince these people that I'm on their side? What would Uncle do? [Impersonating his Uncle, pacing and holding up his finger] Zuko, you must look within yourself to save yourself from your other self. Only then will your true self reveal itself. [Dropping the impersonation and getting frustrated] Even when I'm talking for him I can't figure out what he means!
Deep South: All the swampbenders have hillbilly accents and seem adamantly opposed to wearing pants/shirts/shoes.
Defanged Horrors: This show has many scenes and creatures that are scary even to adult viewers, but stay clear of blood or violence, or monsters jumping at you from the screen. It also portrays a century long war quite realistically and does not shy away from discussing (and within limits showing) genocide and war crimes against civilians, while still remaining safe to watch for children.
Prisoners never seem to be executed. It's claimed by one Earth Kingdom soldier that the Fire Nation sometimes dresses prisoners of war in Fire Army uniforms and sends them unarmed to the front lines, essentially executing them by friendly fire, but it's never made clear if this is actually true.
Defense Mechanism Superpower: The Avatar State is explicitly identified as a defense mechanism by Roku, though it's also possible for an experienced Avatar to enter it at will. Aang doesn't quite master this until the very end of the series. Well, he tried in the second season finale, but that didn't go so well.
Defrosting Ice Queen: Mai in Season 3. She still acts plenty frosty, but is actually capable of showing emotion when either a) Zuko is around or b) it's just her and Zuko.
Deliberately Monochrome: In "The Siege of the North" arc, when the moon spirit is killed, everything becomes black and white, except when lit up by light from fire or water bending.
Demoted to Extra: Teo, The Duke, and Haru in late Season 3. At the end of "Day of Black Sun" they escaped on Appa with the rest of the gAang... according to the commentary, because the writers felt that sending children to a Fire Nation prison would be too harsh. But the writers couldn't figure out anything else to do with them, despite Teo being a great glider pilot and inventor and Haru being a decent Earthbender in his own right… so they only appeared in the background, or the focus of "road trip with Zuko" episodes was set away from the Western Air Temple where they were encamped, until they could be Put on a Bus (along with the just-rescued Hakoda) in "The Southern Raiders". (In later commentary, the writers joked about wanting to do an episode entirely about their adventures exploring the Western Air Temple.)
The lion-turtle was a type 2. Although in Book 2, The Library, there was a set up for the lion-turtle. If not for the fact that this line was said just over a year before the finale was released this could be a type 3. But it is more than likely a type 4.
The energy-bending was a type 1. Sort-of. The information regarding it was given by the Lion-Turtle. Although no prior mention of energy-bending existed there was prior mention of the Lion-Turtle and the creators confirmed that they had planned for that ending (if not the specific mechanism) from early on.
Die or Fly: Used twice. Toph Invokes this by rolling a boulder at Aang in an attempt to get him to learn earthbending. Aang jumps out of the way to keep from being squished. The scene is mirrored later in the episode, when he has to defeat an angry Saber-Toothed Moose Lion, and successfully gets the "stand your ground" mindset required for earthbending. Season 3 has a semi-straight example: Katara learns Bloodbending on her first attempt to stop Hama from making Sokka kill Aang.
Toph's enhanced senses and earthbending prowess as a result of her blindness.
Often lampshaded and parodied with lines such as "Your feet need their eyes checked", "I'll tell your feet what's going on", and "Not being able to see with your feet stinks!"
Also, the Mechanist's son Teo is paraplegic, but has a glider attachment to his wheelchair that lets him fly almost as well as Aang. Though unlike Toph, this flight ability is never overtly suggested to be because of his handicap.
Not the best example since it takes place almost at the end of the series. The finale of Season 1 would be a better example; marking a significant victory for the good guys but not one that actually changes the status quo of the war in general.
Disc One Nuke: The trope is toyed with after Aang finds out about the coming comet; he ALMOST learns fire-bending (sequence breaking the order he is supposed to learn the elements). He's actually quite powerful with just a small bit of instruction, but after he loses control and hurts Katara, both he and his teacher decide he is not ready.
Also happens here and there in the series; once when Appa and the Porcupine-Boar raise hell with their fight in "Appa's Lost Days", and in the beginning of "The Southern Raiders"; Disturbed Doves flying off the water fountain is what warns Aang that something is wrong, about half a second before they are bombed by Azula's Airships. Most notably in the finale, in which an entire fleet of birds scatter off from just about everywhere when Ozai begins his attack. To be fair though, he was burning down their entire forest...
Drawn to the forefront when they steal Fire Nation clothes, she punches her feet through the shoes to remove the soles.
This bites her in the ass (or the soles, if you will) just after Zuko's Heel Face Turn (like, right just after), when she spooks Zuko at night in the woods, then doesn't respond when a wary exile with no one in the world he can trust calls out "who goes there". Predictably, the lack of response makes Zuko assume he's under attack and try to scare off his attacker by scorching their boots; boots which you may remember Toph doesn't wear.
Doomed Hometown: The Southern Air Temple for Aang. The Northern, Western, and Eastern ones did not fare too well either.
Double Consciousness: Being both the son of the Fire Lord and the destined ally of the Avatar has got to be tough.
Don't forget being the great-grandson of both the man who started the war and the first to speak out against it (that is, the past life of the person he worked years to capture).
Double Edged Answer: Toph tries to explain how her feet got burned: she surprised and startled Zuko in the middle of the night and he reflexively flung fire at her before he realized it was one of the Gaang. He was immediately contrite and apologetic, but Toph, in pain, bailed.
Toph: Well, he did and he didn't.
Downer Ending: Several episodes of Season 2, most significantly the Season 2 finale.
Also, an in-series example: The ending of the Ember Island players' play (where the Avatar is killed, the Fire Nation wins the war and Ozai and Azula take over the world) was a downer ending for the Gaang.
The Dragon: Zhao in the first season, Princess Azula in the second and third; she was even portrayed as a blue dragon within a hallucination her brother was experiencing.
...while their uncle was portrayed as a red dragon.
Drama Bomb: The second season finale is infamous because of this. There had been a gradual build up of conflict and drama in the episodes leading up to it, but it all came to a head in that episode with the force of a nuclear explosion.
Dramatic Thunder: First type in "The Storm" and second type "The Southern Raiders"
Guru Pathik makes a short appearance in 'Appa's Lost Days' well before meeting Aang. Hakoda also appears in the same episode, but only for a few seconds.
Also, Azula is seen in the Storm flashback of how Zuko got his scar.
The benders seen in the opening credits (aside from Aang) are not introduced for some time:
the waterbender is Pakku,
the earthbender was the original design for Toph before the creators decided to make her a girl instead (which got reused for Avatar Roku's earthbending teacher). It is also speculated that the Earthbender is the Boulder.
the firebender is Azula
Roku also appears in the opening credits despite not being named until three episodes in, and he didn't even get any lines until four episodes after that.
A vision of Toph in "The Swamp".
Imagery of the Lion Turtle has constantly reappeared throughout the entire series, but never in a way that would make its true purpose apparent.
The married couple that Zuko almost attacks in Zuko Alone reappear as refuges in the Serpent's Pass.
Early Installment Weirdness: Without getting into the differences in characterization, the pilot episode gives Zuko the ability to use flame jets as Reverse Gripped daggers and has Aang consciously activate the Avatar State for a quick boost despite the episode ostensibly taking place sometime during the equivalent of the beginning of the first season. Fire Nation soldiers wear Spikes of Villainy galore and Katara is also named Kya.
Easily Forgiven: Averted hard, especially by Katara. Betraying her in any way will set her default reaction to you to "near-homicidal rage" and it will stay that way unless serious amends are made. The girl can hold a grudge.
Eat the Dog: Sokka offers up Momo to a sea monster as a "humble and tasty" sacrifice.
Eventually, Katara learns how to get water from pretty much anywhere, starting mainly in the third season. She uses her own sweat to escape the wooden cell keeping her and Toph locked up and in the episode "The Puppetmaster" she learns to blood bend and to draw water from nearby plants and even the air itself while empowered by the full moon.
And waterbenders come back with a vengeance because you really dont want to face one in the sea.
This is what makes firebending arguably the most powerful form of bending. You can separate a waterbender from a water source or an earthbender from the earth, but firebenders makes their own fire. Though this could apply to airbenders as well because there's really no way to separate them from their element. And even if that were possible, they'd probably be more concerned with not being able to breathe rather than not being able to bend.
Enemy Mine: "The Blue Spirit" saves Aang from Zhao because neither wants Zhao to take him back to the Fire Nation.
Epic Fail: Hahn, a conceited water tribe leader, uncovers his disguise and charges at Iroh and the Fire General Zhao on their ship. What happens? Iroh and the General Zhao just step out of the way, Zhao easily flings him overboard, and Hahn falls in the water. Then Iroh and the General resume their merry war conversation as if nothing happened.
Ermine Cape Effect: Bumi wears his robes all the time, unless he's showing off how great an earthbender he is.
For the Fire Nation, the fancy robes are often war armor, so it often makes sense to wear them a lot.
Also justified for Bumi, since his basic political strategy relies on convincing people he's a demented old figurehead-until suddenly it's time to show them that he's actually a demented old earthbending master, and also replace "demented" with "alarmingly savvy".
Escalating Brawl: Subverted. Hakoda tries to cause a prison riot by shoving the stereotypical huge tattooed guy... who responds with words because he's working on controlling his anger.
Also, Sozin left Roku to die on a volcano after helping him contain the eruption, around the time it was clear Roku had inhaled too much of the fumes to escape on his own. The look of betrayal on Roku's face says it all.
Iroh feels this way after Zuko betrays him for Ozai's approval.
Evasive Fight Thread Episode: "City of Walls and Secrets" for Zuko and Jet. Even Bad men love their mamas Ursa was a very good mother to Zuko and he misses her deeply.
Even Evil Has Standards: in "Zuko Alone", Zuko is starving and sees a camp set up by the side of the road, with a roast on the fire. He reaches for his dagger. He then sees that the camp belongs to a young refugee couple, the wife of which is heavily pregnant. He takes his hand off his dagger and leaves.
In his backstory, the incident that resulted in his scar started because he objected to sacrificing a division of new recruits for tactical expediency, despite not necessarily opposing the war.
Possibly subverted by Azulon, who objects to Ozai callously asking him to make him successor instead after the death of Iroh's only son. Azulon, if Ozai and Azula are to be believed, then went over the moral event horizon by telling Ozai to kill Zuko.
Exposed to the Elements: Averted with Sokka and Katara, who wear heavy fur coats at both poles. Played straight with Aang, who never seems to be cold even though he always wears the same outfit (though this is Hand Waved by him knowing a special breathing technique). Averted and Justified with Zuko, who is seen using a special technique that allows him to breathe fire. He also dresses appropriately and is in very real danger of freezing to death after he gets knocked unconscious, prompting Aang to take him with them.
Zuko served as this to Katara in Season 1. Both lost their mothers at a young age, lost contact with their fathers for several years, and were forced to grow up very quickly as a result. Also, both have serious anger issues. Even their families are a twisted mirror of each other: a distant father, a lost mother, and a sibling who is eager to fight. Adopting each other's strengths in order to overcome their individual weaknesses helps them to grow as people.
Zhao was the Eviler Counterpart of Zuko in Season 1. Both favored power over control with their firebending and obsessed with gaining honor and reputation. Zhao was so obsessed though that it actually led to his Moral Event Horizon.
Similar in effect to Jet, Hamma served as this toward Katara; harboring deep hatred towards the Fire Nation because of what they did to her in her childhood. Hamma's unwillingness to let it go led to her becoming no better than the ones who persecuted her.
Azula became this toward Zuko in Season 3. Both suffered Parental Abandonment, although in Azula's case, it may only be from her perspective. Zuko however was able to get a Parental Substitute in Iroh while Azula was shaped by other parent, the super-evil Ozai.
For the ultimate example: Sokka, Katara, Hakoda and Kya versus Zuko, Azula, Ozai and Ursa. Their whole family is a counterpart: the older brother struggling to live up to his father's position, but overshadowed by the talents of his little sister, the bending prodigy; meanwhile, both siblings are still affected by the loss of their mother several years ago. Except Zuko's father is an unpleasable psychopath who exiled him, while Sokka's father is wise, kind and had to leave Sokka behind; Katara is hot-tempered but a total Team Mom, while Azula is cold-hearted and controls people with fear; and Kya sacrificed her life to protect her daughter, while Ursa murdered an old man in cold blood to protect her son (though, she also had to give up her life too, in the form of banishment, so she probably serves more as a Shadow Archetype).
Evil Is Burning Hot: The Fire Nation is initially played up to be this, but it's later subverted in that some become allies to Aang.
Evil Me Scares Me: Aang actually says, "I was scary" when reflecting on the Avatar State. A little different since the Avatar State isn't evil but rather an implacable force of nature.
Evil Old Folks: Fire Lords Sozin and Azulon, though the former came to regret his decisions and at least believed (or convinced himself) he was working for the good of the world. Ozai may or may not count, depending on how old he is. A few other Fire Nation villains are a respectable age too. Non-Fire Benders include Old Lady Hama, the vengeful and deranged Water Bender who abducts innocent Fire Nation civilians and locks them in an underground cave on the basis that they were born in the wrong country.
Evil Prince: Ozai was a classic example of this trope before he became Fire Lord. Subverted with Zuko, who ends up trying to overthrow his father not to seize power for himself, but to end the war. Azula is a complete aversion of this, because despite being evil, she remains completely loyal to her father.
Eviler Than Thou: Zuko was plenty threatening during the first season, but Azula is superior in just about every way (except, you know, morals). She also manages to completely outmaneuver Long Feng in the Season 2 finale.
Expository Hairstyle Change: Most major characters went through hairstyle changes over the course of the show, in stark contrast to many cartoons' habit of keeping hairstyles exactly the same to make it easier to animate. There were so many hairstyle changes, in fact, that it gave rise to the fan term "hairbending".
Zuko's hair changed from an exotic, Samurai-inspired shaved-with-a-topknot look in Season 1 to a shaggy mop by the end of Season 2 while his character becomes less and less the villain and more and more three-dimensional. Said mop was thus long enough to tie back into a dignified Fire Nation topknot just in time for Zuko's coronation.
Katara's hairstyles in Season 3 were more elaborate than the ones she wore in Season 1 and 2, making her appear more mature… matching the way she grew as a "team mother" over the course of the show.
For the first two thirds of Season 3, Aang actually had hair (having been too busy being unconscious to shave, and determining it made a good way to disguise his head tat while sneaking around in the Fire Nation afterward).
Eyes of Gold: The Fire Nation people. Even Evil has loved ones . Prince Zuko while not verry good at expressing it loves his Uncle and priorotises Iroh,s safety over his mission to find the avatar these feelings are reciprocated and Iroh does what he can to guide his nephew on the right path. Mai and Zuko serve as this to each other as boyfriend and girlfriend and it is ultimately Mai,s love for Zuko which caused her to betray Azula . Mai,s uncle is a ruthless prison Warden and harbours particular dislike for Zuko because he broke his neice,s heart. The Warden also used his connections to get Mai released from Prison. Firelord Azulon was a near absolute Psychopath but he refused to betray his firstborn Son and attempted to punish Ozai in the worst way imaginable . While Iroh is mostly benevolent these days in his past he was a ruthless War General who found the idea of Ba Sing Sei,s destruction numerous in spite of this He loved his son Lu Ten which Caused him to lose his desire For Power and Focus on the good things in life .
In a memorable moment in Season 2 there was a sick Zuko in bed sweating and shirtless begging for water, when Iroh finally gives him a taste, he then takes the container and basically pours it all over himself.
Air Nomads: Tibet, some elements of the Shaolin Temples of China.
Water Tribes: Inuit, with a little Pacific North West Indian (Northern Tribe).
Earth Kingdom: Qing (Manchu-dominated) Dynasty China with a little bit of Korea (Song's family from "The Cave of Two Lovers", Ba Sing Se's...unique worldview) and Japan (Kyoshi Islands) in the mix.
Peasant Clothing in the rural parts of Earth Kingdom is predominately Korean hanboks.
Fire Nation: Tang Dynasty China/Taisho (Imperial) Japan/Mongolia. There are also Indian touches (agni kai, for example).
The original fire-bending civilization, the Sun Warriors, borrow from Native American cultures, specifically Aztecs and Mayans, but are predominately based on the Candi Sukuh of Indonesia, with architectural designs coming from Angkor Wat and Phanom Rung and headdresses resembling those of Iban warriors.
Fantasy World Map: The official Avatar site at nick.com has a nice world map documenting the Gaang's travels during the series.
Fate Worse Than Death: Zuko and Iroh, while on the run as fugitives of the Fire Nation, were left with a choice: seek help in the Earth Kingdom, which mostly views all firebenders as war criminals and would likely be killed; or return to the Fire Nation, where they would be captured by Azula. Zuko quickly decides on the former.
Ozai's receiving Cruel Mercy can be considered one after losing his ability to firebend.
Sokka: Come on, a day at the theater? This is the kind of wacky time-wasting nonsense I've been missing!
Not abundant in the first two books, but jarringly common in Book 3. It's especially odd feeling because many filler episodes are put right in the middle of what would normally be the darkest arc of the series (or even worse, right before the finale like 'Ember Island Players').
Flynning: A deliberate, justified case on the part of one character. In Sokka's Master, Sokka and Piandao have a swordfight that lasts about three minutes. on the DVD Commentary, Sifu Kisu (the martial arts director) says that if this were a real swordfight, it would last under five seconds, but Piandao is just testing Sokka.
Foe-Tossing Charge: In the second season finale. After Aang is shot with lightning and is falling to the ground, Katara rides a gigantic wave over Zuko and the Dai Li to catch him.
After running away from the pressure of being the Avatar, then being frozen for 100 years, Aang awakened in front of Katara. At first, he fell in Love At First Sight and all he wanted to do was play games or show off to her, all the while unaware of what he should be doing. Hanging on her every word, he gets on the right track in the Southern Air Temple. He finds friendship and camaraderie all around him, and comes to love the world selflessly. But it's always Katara he loves the most.
Zuko hunts the Avatar all around the world for his father's affection. When he returns to his father in Book 3, he not only regains a sense of belonging, he enters a happy relationship with Mai. But he later realizes his uncle Iroh loves him more than his father ever would. In the end, he comes to love and respect all four nations, is reunited with Iroh, keeps the Gaang as his first true friends, and gets to stay with Mai.
Free-Range Children: A Deconstruction. The kids have free reign to go on adventures because, with the exception of Toph, their parents are either dead or busy fighting in the war.
Gadgeteer Genius / Reluctant Mad Scientist: The Mechanist from "The Northern Air Temple" and "Day of Black Sun" is a classic lift right out of the Steampunk genre, a highly eccentric genius with steam and mechanisms who reluctantly lends his talents to making weapons for the Fire Nation when they threaten his home and people.
Sokka also shows some signs of inventive talent while at the temple and the submarines from "Day of Black Sun" were also his idea, and his father invented the "stink and sink" mine.
Gaia's Vengeance: The ferocity in which Aang attacked "Phoenix King" Ozai when he was not himself in the Avatar State indicates that the World itself (with Aang as its Avatar) wanted Ozai dead in the worst way.
Hei Bei is a localized example. After the Fire Nation burned down an entire (assumably ancient) forest, the guardian spirit flies into a rage and terrorizes the (innocent) population of the countryside until the Aang shows it the acorns that will regrow the forest in time.
For all intents and purposes, this is generally what the Avatar's job is, and even then it's only when it gets that bad.
This is how Toph grew up. She lived in luxury and had the run of the whole estate. But she wasn't allowed to travel outside the estate or exercise her incredible potential at Earthbending--her parents thought this was too dangerous for their "helplesslittleblindgirl." Nobody other than her family and her Earthbending teacher even knew that she existed.
The Gaang's experience in Ba Sing Se. They were allowed to indulge in all the luxury they wanted, as long as they didn't try to leave, or break the rules, or evade the constant surveillance, or search for Appa, or tell anyone about Long Feng's Government Conspiracy or the war with the Fire Nation...
Subverted with Meng, to whom he doesn't give the time of day.
Giving Up on Logic: Sokka never fully does this, but he starts off as almost a Flat Earth Atheist who says that bending is magic and a flying bison could never exist to casually accepting all the various genuinely supernatural experiences the gaang goes through.
A God Am I: While Sozin was more or less an Evil Overlord and nobody saw enough of Azulon to know what went on with him, Ozai fits this during the finale, just by crowning himself Phoenix King before he's even won. The hammiest example of this, however, was Zhao at the North Pole:
Zhao: I am... a legend now. The Fire Nation will for generations tell stories about the great Zhao, who darkened the moon! They will call me Zhao the Conqueror! Zhao the Moon Slayer! Zhao, the INVINCIBLE!
God Guise: One episode has Katara disguising herself as a local guardian spirit (not exactly a deity though), The Painted Lady, in order to attack a Fire Nation outpost and inspire the locals who used to look to the spirit as a patron and guardian. Though she's eventually exposed, the villagers forgive her.
Godiva Hair: Katara has this during her bathing scene in 3x7, "The Runaway", used in conjunction with just staying underwater.
Gone Horribly Right: In the Season 2 opener, General Fong convinces Aang to attempt to use the Avatar State to take on the Firelord without having learned all the elements. After many failed attempts of bringing on this state however, Fong grows impatient and tricks Aang into the Avatar State by pretending to harm Katara. He gets his wish alright, and a good chunk of his fortress is destroyed in the process.
Good Angel, Bad Angel: Zuko's uncle and sister in the Season 2 finale; in the episode before, Zuko goes into an Angst Coma and dreams about a Red dragon that speaks with Iroh's voice and a blue dragon with Azula's. This is a neat foreshadowing of Avatar Roku and Fire Lord Sozin being his implied historical and psychological Good Angel, Bad Angel in The Avatar and the Fire Lord - Roku (Ursa's grandfather) owned a red dragon and Sozin (Ozai's grandfather) owned a blue-green one.
Good Cannot Comprehend Evil: Aang holds this view for the majority of the first season, in keeping with his pacifistic views. Even at the end, when he's facing Ozai, he can't bring himself to kill him.
Good Eyes, Evil Eyes: Zuko's scar shows that the writers and character artists are well aware of this trope.
Aang's eyes are by far the widest of all the characters.
By Season 3 Zuko's eyes become wider and less menacing.
Good Is Dumb: Lampshaded when, Right after getting into Aang's group, Zuko finds his firebending suddenly lacking and guesses, "I bet it's because I changed sides." He had been using anger to fuel his bending up to that point and had learned to let go of it, temporarily leaving him without a proper conduit.
Good Scars, Evil Scars: Zuko's facial burn actually covers 2 variations of this trope. When he is evil, it makes him look that much more menacing, but in the episodes where he is good(ish), it makes him look more noble -- it is actually taken as a cue by Song and Jet that he is a fighter against the Fire Nation.
Green Aesop: The Fire Nation's industrialism seems like this, but it was most clear in "The Painted Lady".
However, it was subverted when the Gaang first meets Teo and The Mechanist.
Groupie Brigade: Aang acquires a groupie brigade of young girls on Kyoshi Island. Then he loses them when they get bored with his half-hearted attempts at showing off.
Guys Smash Girls Shoot: Averted. Benders of both genders freely mix long- and short-range attacks, as does Sokka. Mai might only do ranged attacks, but Ty Lee and Suki are strictly melee fighters.
A more proper divider would be "Muggles Smash, Benders Shoot". It seems if you know any bending, then you can only fight at all with Bending--the only people who throw physical attacks that connect are the non-benders. Any bender will only ever make contact with an enemy with their element except, occasionally, to block.
Except Zuko, who takes out a few Earth Kingdom soldiers without bending in "Zuko Alone" not to mention his escapades as the Blue Spirit
Coinciding with the above statement, although Muggles Smash, Benders Shoot is the better descriptor. In terms of the Gaang this really only applies to Katara who is not at all versed in hand to hand combat and really is at a bad disadvantage when she can't bend or has no water to draw from. Generally however, the more talented benders seem to be rather capable at handing out beatdowns or at least getting by without their element; Zuko [as noted above), Aang (a rarity, but can go on a really good dodging defensive), and Iroh for sure and highly possible for Azula and Toph.
Hands Play in Theater:(regular theater, here)in the episode "Ember Island Players". Played stright with Sokka and Suki (his arm around her shoulders), and averted with Aang and Katara (Zuko sit between them.).
Hard Work Hardly Works: Subverted. It looks as though Aang learns waterbending faster than Katara (which would at least be justified by being the avatar), but when Katara and Aang get an actual instructor instead of relying on self-teaching and the scroll, Katara masters it even faster than Aang. Furthermore, Aang has trouble with earthbending since that is the counter to air. Of course Aang is shown goofing off during the waterbending lessons.
For the brief moments we view Iroh in his prison cell, you gradually see him transform from a badass sack of lard into a even more badass chissled bodybuilder. This also parallel's Sokka's sword training.
He Who Fights Monsters: The series provides two examples of this in Jet and Hama, two individuals whose hatred of the Fire Nation blinded them to the point where they could no longer distinguish between friend and foe. Hama was even worse than Jet, because he at least tried to justify it as doing the right thing by making sacrifices, and he did redeem himself in the end. Hama was treated so badly that she targeted anyone within reach, mostly civilians. Both of them served as a warning to Sokka and Katara about what they could become if they continued to hold on to their own prejudice and anger.
Heel Face Turn: If you watch the first season you will not be too surprised by Zuko turning. But it is played with and at least initially subverted in several instances, the second season finale being the most infamous. Jet Shows regret for his actions and bonds with Zuko seemingly subverted when his prejiduce towards the firenation makes him turn on and attack Zuko but then double Subverted when Jet dies helping Aang fight Long Feng
The spirit Hei Bai is one of the earliest examples, though all it took was reassurance that his forest would grow back.
Heh, Heh, You Said "X": Aang and Sokka snicker when the professor admires the buttresses in Wan Shi Tong's library.
Held Gaze: the series has a few of these. One with Mai and Zuko in the Boiling Rock episodes after being forced to lock the cell door on her [this one is more of deep hurt though], one with Suki and Sokka on the Serpent's Pass leading to an Almost Kiss. Then finally the one between Aang and Katara which leads to the Big Damn Kiss and end of the series.
High Fantasy: An epic that involves the fate of the world, a young boy and his Ragtag Bunch of Misfits who are Walking the Earth, politics that change the course of the world, gods (called spirits but act as deities) that interact with the protagonists and, as a refreshing twist, takes place in a mythical world insipired by Eastern culture rather than Western. It would go in Wuxia save for the fact that it holds very little in common with the genre.
Hook Hand: Actor Jet in "The Ember Island Players".
Hope Spot: Happens in the second season finale when Aang enters the Avatar State intentionally for the very first time and looks ready to wipe the floor with everybody... only to be electrocuted by Azula.
The entire episode right before the last two episodes of the second season. Appa's back, the Earth King has sided with the Gaang, Long Feng has been arrested, and each of the characters have personal issues that are looking up. Unfortunately, Long Feng has allies and is planning in the background, Toph is captured, and Azula, along with Mai and Ty Lee, has disguised herself as a Kyoshi Warrior and about to show how good of a Chessmaster she is.
Hot Amazon: Suki and the rest of the Kyoshi Warriors, but their namesake took it to the extreme, being both beautiful and extremely tall. (She even reportedly has the largest feet of any Avatar so far!)
Hot Dad: Ozai was designed to look like an older bishie-ish, unscarred version of Zuko.
Humanoid Abomination: The Avatar State borders on this, especially during the fight with Ozai in the finale. Since the state is basically allowing a force of nature to act through you instead of yourself, it's much more brutal and ruthless than Aang is normally.
Humiliation Conga: Ozai, quite a bit near the end. Jerk with a heart of gold. Sokka is extremely immature Sarcastic and arrogant but has a protective instinct over Katara and is very loyal to his friends. Zuko also proves to be this perhaps best demonstrated when he claims to not care about the safety of the crew only to risk his life to save one of them demonstrating that while he,s not always the most likeable guy he will chose to do what,s Right when it counts . Knight Templar . Jet belived fully that he was on the right Side despite endangering innocents . Mama Bear Ursa would do anything to protect her son including conspiring with her husband to Kill his father . Noble Demon. Zuko Values things such as Honor and Loyalty and even attempted to save his more evil Rival Zhao . Pet the Dog. Zuko gets Many of these Moments such as releasing Appa with Iroh,s persuasion Standing up for an earth Kingom Family against bullying Soldilers and abandoning His plans to steal from a couple upon Realising the couple was pregnant Zuko also Shared one with Jet when they stole from a greedy Captain to feed the rest of the passengers on the ship .