Which Assassin's Creed game introduced Ezio Auditore as its main protagonist?

Assassin's Creed II introduced Ezio Auditore as its main protagonist.

Released in 2009, the game follows Ezio from his youth in Renaissance Italy through his transformation into an Assassin. It is the direct sequel to the 2007 original, whose protagonist was Altaïr Ibn-La'Ahad.

Assassin's Creed II expands the series beyond the original game's Middle Eastern setting. Its story visits Italian locations including Florence, Monteriggioni, Venice, and Forlì, while fictional characters interact with historical figures such as Leonardo da Vinci and Rodrigo Borgia.

Ezio later returns as the central character in Assassin's Creed: Brotherhood and Assassin's Creed: Revelations. Those games are sometimes grouped with Assassin's Creed II as the “Ezio trilogy,” but they are separate releases. The game also established many systems that became series staples, including economic upgrades, a larger variety of weapons, and Leonardo's mechanical inventions.