Bayek of Siwa becomes the first Mentor of the Hidden Ones in Assassin's Creed Origins.
Bayek and Aya establish the Hidden Ones after confronting the Order of the Ancients, the organization that opposes them in ancient Egypt. Their goal is to create a group that protects ordinary people from those who use political power and violence for control. Bayek serves as the order's first Mentor, while Aya later operates under the name Amunet.
Origins presents this organization as the direct predecessor of the later Assassin Brotherhood. The title “Mentor” therefore describes a leadership role in the early Hidden Ones, not a rank that began in the original 2007 game.
The story takes place during the reign of Cleopatra VII and includes conflicts involving Julius Caesar. Bayek's Medjay identity is important to his personal history, but it is distinct from his later position as Mentor. Aya's role is equally significant, although the question asks specifically for the first holder of the title.