Desmond Miles is the Animus user who relives memories in the first Assassin's Creed game.
Desmond is a modern-day descendant of several Assassin ancestors, including Altaïr Ibn-La'Ahad. Abstergo Industries, a modern corporation secretly aligned with the Templars, abducts him and places him in the Animus. The machine allows him to experience Altaïr's genetic memories while Abstergo searches for information connected to an ancient artifact.
The first game alternates between Desmond's 2012 storyline and Altaïr's experiences during the Third Crusade. Although players spend most of the historical gameplay controlling Altaïr, Desmond is the modern protagonist who makes the memory sequences possible. His role expands across the early games and connects separate historical protagonists within one continuing plot.
William Miles is Desmond's father and a senior Assassin, while Warren Vidic is the Abstergo scientist overseeing the Animus project. Clay Kaczmarek is another modern-day subject in the broader series, but none of those characters is the user in the original game's central Animus setup.