In Assassin's Creed II, Ezio Auditore's uncle who helps him after his family is targeted is Mario Auditore.
Mario is the head of the Auditore family branch in Monteriggioni and the owner of Villa Auditore. After Ezio's father and brothers are executed in Florence, Mario arrives with his men and takes Ezio and his sister Claudia to safety. He then begins teaching Ezio how to fight and survive in the political violence surrounding the conspiracy.
Mario is more than a helpful relative: he is an experienced fighter and an important supporter of the Assassin cause. Monteriggioni becomes Ezio's base, and the villa can be rebuilt and upgraded during the game. This makes Mario easy to confuse with other historical or family figures in the story, especially Lorenzo de' Medici and Niccolò Machiavelli, who assist Ezio at different points but are not his uncle.
Mario's role connects Ezio's personal revenge to the wider Italian Assassin network.