Which Assassin's Creed game introduced the playable character Altaïr Ibn-La'Ahad?

Assassin's Creed introduced the playable character Altaïr Ibn-La'Ahad.

Altaïr is a Syrian Assassin whose story is set mainly during the Third Crusade in the late 12th century. After a serious failure, he is demoted by his mentor Al Mualim and must complete a series of investigations and assassinations to regain his rank. The game establishes many recurring series ideas, including social stealth, Eagle Vision, parkour and the conflict between Assassins and Templars.

Altaïr also appears in later games, especially Assassin's Creed Revelations, which revisits his life and legacy. That later appearance can lead to the mistaken belief that Revelations introduced him. The original 2007 game, however, is the title that first presents Altaïr as a playable protagonist.