A character who chooses a codename for him/herself.
While this MAY seem similar to the recently-launched trope "Self Applied Nickname" on TV Tropes, that trope is a Comedy Trope about a character who is never referred by a nickname (either in-universe or out) and who insists on a nickname that never catches on.
This is when the trope is played dead serious, as an Establishing Character Moment. Once the character chooses the nickname, he or she will mostly be referred to by the nickname, and even referred out-of-universe by the nickname.
Unlike the nicknames in "Self-Applied Nickname" (which tend to be humourous to the point of ridicule), the nicknames in this trope are ominous and imposing, with a sense of flair and elegance. In fact, this trope often occurs when a character is saddled with an more embarassing nickname.Comic Book Movies Don't Use Codenames, by having the movie character choose what is his/her usual codename in the comics.
- The Trope Namer is Adrian Veidt from Watchmen. After he became fascinated with Ramses II, he choose the codename "Ozymandias" for himself. After that point, everyone calls him Ozymandias.
- The trope image depicts the Ur Example of this, when Bruce Wayne chooses the codename "Batman" in order to strike fear deep into the heart of criminals. This codename would last him a lifetime.
- After being saddled with the rather unfortunate codename of "Paste-Pot Pete" for most of his career, Peter Petruski chose the codename "The Trapster" for himself. While some comedic moments have other refer him by his former nickname, most of the time he's referred as the Trapster.
- Negasonic Teenage Warhead. While the characters openly admit how over-the-top it is, nobody calls her "Ellie Phminster".
- In the movie Noise, the Vigilante Man protagonist chooses the codename "The Rectifier" for himself.
- Downplayed with the Saw movies. While John Kramer didn't come up with the nickname "Jigsaw", he DID come up with the reason behind that nickname.
- Neo refuses to be adressed as "Thomas Anderson", and outside of Agent Smith, everyone else calls him Neo.
- Karr in Bulletproof Monk gave himself that name.
- Santanico Pandemonium chose this name for herself shortly after becoming a vampire.
- In addition to coming up with the codename "The Saint" himself (much like in the novels and TV show), in the movie he also comes up with this "actual" name, Simon Templar.
- Francis Dolarhyde came up with his codename "the Red Dragon". The comedic overtones of Self-Applied Nickname get deconstructed here: a reporter came up with the nickname "The Tooth Fairy", which everyone uses to refer to Dolarhyde. Dolarhyde kidnaps said reporter, tortures the reporter while shouting that his (Dolarhyde's) codename is "Red Dragon", then burns the reporter alive while leaving a tape in the reporter's corpse that sets the record straight on Dolarhyde's codename being "Red Dragon".
- The Count of Monte Cristo chose this name in order to ennact his revenge scheme against the man who ruined his life.
- Once he chooses the codename "Sylar" for himself, almost nobody refers to the character as Gabriel Grey anymore.
- After his sister mistook him for an angel when he was first sired, Liam took the name Angelus, and later his ensouled self took the name Angel. Nobody on either Buffy or his show ever refers to him as "Liam".
- The Doctor chose his name for himself. His real name is unknown to almost everybody, so much so that it's a plot point.
- Anton Ivanov in Agents of S.H.I.E.L.D. chose the codename "Superior" for him.
- The Rat King in the 2012 version of Teenage Mutant Ninja Turtles, much to Michelangelo's irritation, since he's the one who usually comes up with the nicknames for the mutants.
- Electro in The Spectacular Spider-Man, to be point of flying into a rage if he is ever called "Max Dillon".
- After being horribly disfigured by a rampaging Ursa Major, The Dragon in My Little Pony: The Movie chose the name "Tempest Shadow" for herself.
- Hardly anyone knows or cares that Mojo Jojo's real name is just "Jojo".
- In Transformers Animated, Scrapper and Wreck-Gar chose those names for themselves.