Which Assassin's Creed game introduced the Animus's modern-day protagonist Desmond Miles?

The first Assassin's Creed game introduced Desmond Miles as the modern-day Animus protagonist.

Desmond is a bartender descended from several important Assassin ancestors. Abducted by Abstergo Industries, he is forced to use the Animus, a machine that lets him relive genetic memories. His first major memory is that of Altaïr Ibn-La'Ahad, the game's medieval protagonist.

This dual structure—modern-day scenes surrounding a historical simulation—became one of the series' defining storytelling devices. Desmond's modern storyline continues through later entries, including Assassin's Creed II, Brotherhood, Revelations, and III.

The distinction matters because Assassin's Creed II introduces Ezio as a historical protagonist, not Desmond himself. Desmond remains the person outside the Animus in that game, while Ezio's life is the memory being experienced.