Desmond Miles is the Animus user who relives ancestral memories in the original Assassin's Creed.
At the beginning of the series, Abstergo Industries abducts Desmond and places him in the Animus. The machine allows him to experience genetic memories inherited from ancestors, including Altaïr Ibn-La'Ahad in the first game. Desmond's modern-day scenes provide the frame narrative around the historical missions.
Desmond is not the historical Assassin being controlled in those sequences. Altaïr is the ancestor whose memories Desmond experiences, while Warren Vidic supervises the experiment for Abstergo. This distinction is central to understanding the series' dual timelines.
Desmond's family also connects him to the wider Assassin Brotherhood. William Miles is his father, and Shaun Hastings is an Assassin ally introduced in later modern-day sections. Desmond remains the primary modern-day protagonist through Assassin's Creed III, where his arc reaches its major conclusion.