Desmond Miles is the Animus user and modern protagonist in the first Assassin's Creed.
Desmond is captured by Abstergo Industries and forced to use the Animus, a device that reconstructs ancestral memories. In the first game, he relives the memories of Altaïr Ibn-La'Ahad, allowing Abstergo to search for information connected to an artifact called the Apple of Eden.
Desmond's storyline supplies the modern framework for the historical missions. He later experiences memories connected to Ezio Auditore and other ancestors in the early Assassin's Creed games. His father is William Miles, a senior figure in the modern Assassin Brotherhood.
Clay Kaczmarek is another Animus subject whose memories appear in the series, while Daniel Cross is a Templar agent linked to the modern conflict. Neither is the first game's central Animus user. The name Desmond is therefore the precise answer when the question refers to the original game's modern character rather than its historical protagonist.