Leonardo da Vinci is considered to be one of the best painters that ever lived and one of the most talented individuals to ever walk the face of the Earth.
If anything, Leonardo can be most remembered for his Mona Lisa painting. However, focusing on just one of his creations would mean dismissing an enormous amount of spectacular inventions and paintings. Leonardo often experimented with new techniques and methods, which most likely lead to the destruction of some of his works. Despite this and his habit of procrastinating, Leonardo went on to "invent" the helicopter, tank, calculator, the theory of plate tectonics, solar power, and more. However, due the the limitations of the technology of his era, most of these were only conceptualized and were unable to be properly realized until hundreds of years after Leonardo's death.
In Assassin's Creed II and Assassin's Creed: Brotherhood Leonardo frequently aids Ezio Auditore in creating and improving gear and decoding the codex. In Assassins Creed II he moves from Florence to Venice and finally to Monteriggioni towards the end of the game. In Brotherhood he can be found on benches marked with the letter L to buy extra equipment from. While Ezio recovers from the attack on Monteriggioni, Templars capture and interrogate Leonardo, forcing him to build war machines which Ezio must destroy during side missions. As Leonardo stays behind in Italy during Ezio's trip to Constantinople, he plays no role in Ezio's final adventure in Assassin's Creed: Revelations.