In Assassin's Creed, which 12th-century Assassin does Desmond Miles relive through the Animus?

In Assassin's Creed, Desmond Miles relives the memories of Altaïr Ibn-La'Ahad through the Animus. Altaïr is the playable protagonist of the first main game, set largely during the Third Crusade in the Holy Land.

Altaïr begins the story as a highly ranked Assassin, but a failed mission causes him to lose his rank and equipment. His mentor, Al Mualim, assigns him a sequence of targets in cities including Jerusalem, Acre, and Damascus. These missions gradually reveal a wider political and supernatural conflict.

Altaïr is often confused with Ezio Auditore, the protagonist of the next major game, Assassin's Creed II. Altaïr is also distinct from Desmond: Desmond is the modern-day descendant whose genetic memories are being accessed. The character's name is sometimes written with different apostrophe and diacritic conventions in English-language sources.