Desmond Miles experiences Altaïr's memories through the Animus in the original Assassin's Creed.
Desmond is a former Assassin descendant who is abducted by Abstergo Industries. The company forces him to use the Animus, a device that reads genetic memories, so researchers can search his ancestry for information connected to an ancient artifact.
The historical sequences take place through Desmond's genetic link to Altaïr Ibn-La'Ahad, an Assassin active during the Third Crusade. The modern-day sections at Abstergo frame the historical gameplay and establish the series' long-running conflict between Assassins and Templars.
William Miles is Desmond's father and a later character in the series. Warren Vidic supervises the Animus project, while Clay Kaczmarek is another modern-day subject introduced in later entries. None of them is the first game's central Animus user.