In Assassin's Creed, the protagonist investigating Altaïr Ibn-La'Ahad's memories is Desmond Miles.
Desmond is a modern-day descendant of the Assassin Altaïr. After being captured by Abstergo Industries, he is placed in the Animus, a machine that reconstructs ancestral memories through genetic information. The device allows him to experience Altaïr's life during the Third Crusade.
The historical sections follow Altaïr in 1191, while Desmond's contemporary storyline reveals Abstergo's interest in locating powerful artifacts. This two-period structure became one of the defining features of the Assassin's Creed series. Desmond is therefore not the historical character controlled for most of the first game's missions, even though he is the modern protagonist.
Later games continue to use Desmond as a central viewpoint character, especially Assassin's Creed II, Brotherhood, and Revelations. Other games introduce different modern-day characters or use more indirect present-day perspectives, but Desmond remains the series' best-known early protagonist.