Leonardo di Ser Piero da Vinci (can we call him Leo?) is an Italian scientist, mathematician, engineer, inventor, anatomist, painter, sculptor, architect, botanist, musician and writer. Born in 1452 in Vinci, Italy (hence da Vinci, i.e. from Vinci). Leonardo apprenticed at the art studio of Verrocchio. According to one legend, when Leonardo and Verrocchio collaborated on a painting, Verrocchio found himself so outclassed by his apprentice that he abandoned art altogether.
Leonardo spent time in Florence (where he got charged with sodomy, sparking centuries of speculation over whether or not he was gay), Milan and Rome, working for various wealthy patrons. He produced some of the most famous paintings of Western civilization: The Last Supper, Virgin of the Rocks, and, most famous of all, Mona "what-is-she-smiling-at" Lisa, considered to be either the most beautiful or the most kitschy work of art in history.
Besides above-ground art, Leonardo also trafficked in the shady world of anatomy, secretly procuring corpses and dissecting them (an act which could get you killed in that time) to study the human body.
Leonardo always considered himself foremost a scientist. The areas of his interest include: aeronautics, anatomy, astronomy, botany, civil engineering, chemistry, geology, geometry, hydrodynamics, mathematics, mechanical engineering, optics, physics, pyrotechnics and zoology (allowing him to put pretty much every Omnidisciplinary Scientist in fiction to shame). His inventions range from the mundane (strut bridges, automated thread-winder, pulley systems) to the height of--for the times--science fiction, including tanks, airplanes, helicopters, and a mechanism for walking on water. Leonardo's awesomeness is such that he even gets his own genre of science fiction--Clock Punk.
In fact, it is his interest and expertise in so many areas inspired the term "Renaissance Man". Unfortunately, though, he was interested in so many things that he couldn't settle down to work on anything. Many of his artworks and engineering projects never got off the drawing board, or, in the artwork's case, the doodlepad.
Leo died in 1519. He probably was not an immortal alien...or a Time Lord...or a sacred feminine worshiping cryptologist...or an ally of an order of assassins. And he was definitely not a ninja turtle.
Also, please note that people called him Leonardo (NOT da Vinci) back then. Da Vinci means "of Vinci", so it would be somewhat like saying "What would Of Nazareth do?"