The Assassin's Creed protagonist who uses the alias Bayek of Siwa is Bayek.
Bayek is the central character of Assassin's Creed Origins. He is a Medjay, a protector and lawkeeper from the Siwa Oasis, and his personal search for the people responsible for his son's death draws him into the political struggles of Ptolemaic Egypt. The title “of Siwa” identifies his home and distinguishes him from other characters.
Bayek's wife, Aya, is a major character and an important founder of the Hidden Ones. His eagle Senu assists with reconnaissance, while Hepzefa is a close friend from Siwa. These names can make short references to Bayek confusing, but they are separate characters and do not replace him as the protagonist.
Origins presents Bayek as an ancestor of the later Assassin tradition rather than as a member of the already established historical Brotherhood. His role as a Medjay connects the game's personal story to older Egyptian traditions of service and protection.