In Assassin's Creed Revelations, which legendary Assassin is Ezio searching for in Masyaf?

Ezio is searching for Altaïr Ibn-La'Ahad in Masyaf in Assassin's Creed Revelations.

Revelations brings together Ezio's late-life journey and Altaïr's legacy. Ezio travels to Masyaf, the historic headquarters of the Levantine Assassins, seeking keys that unlock Altaïr's hidden library. The search leads him through Constantinople and into memories connected with Altaïr's leadership of the Brotherhood.

The game is designed as a bridge between the first Assassin's Creed and the Ezio trilogy. Through the Animus, Desmond Miles also experiences parts of the story, creating a three-generation structure involving Altaïr, Ezio, and Desmond.

Masyaf is especially associated with Altaïr because it was the headquarters of the Levantine Brotherhood during his era. Bayek, Edward, and Ratonhnhaké:ton are protagonists from other historical periods. The question asks for the person Ezio seeks, not the location or the artifacts he must collect.