Desmond Miles is the main modern-day playable protagonist in the first Assassin's Creed game.
Desmond is a former Assassin descendant who is captured by Abstergo Industries and placed in the Animus. Through the machine, he relives the memories of Altaïr Ibn-La'Ahad, the game's principal historical character. Desmond's perspective supplies the modern setting while Altaïr's memories provide most of the action.
Players sometimes answer Altaïr because he is the character controlled for the historical missions. The distinction matters: Altaïr is the historical protagonist, while Desmond is the modern-day frame protagonist. Desmond continues as a central modern character through Assassin's Creed II, Brotherhood, Revelations, and Assassin's Creed III. His father, William Miles, and Abstergo researcher Warren Vidic are important supporting figures, but neither is the first game's modern playable lead.