In Assassin's Creed Origins, the Medjay who leads the main story is Bayek of Siwa.
Bayek is a protector from the Siwa Oasis in Ptolemaic Egypt. The murder of his son Khemu drives his personal quest and brings him into conflict with a group of masked conspirators. Alongside Aya, his wife and a major character in the story, Bayek helps lay the foundations of the Hidden Ones.
Origins is set before the Assassin Brotherhood has taken the form seen in later games. Its narrative connects Bayek and Aya's experiences to the creation of that organization. The game also introduced a role-playing structure, level-based enemies, extensive exploration, and a large Egyptian landscape that includes Alexandria, Memphis, and the Giza plateau.