In Assassin's Creed Origins, what is the name of the Medjay who becomes an Assassin?

The Medjay who becomes an Assassin in Assassin's Creed Origins is Bayek of Siwa.

Bayek is a Medjay, a protector and lawkeeper in ancient Egypt, and the game’s central protagonist. His story begins in 49 BCE, during the final years of the Ptolemaic Kingdom. After a personal tragedy, Bayek and his wife Aya pursue members of an underground order whose actions threaten Egypt’s political and social stability.

Origins depicts the formation of the Hidden Ones, the organization that later develops into the Assassin Brotherhood. Bayek is therefore an early founder figure rather than a member of the already established medieval order seen in the first game. “Of Siwa” identifies his home oasis in Egypt and distinguishes him from later protagonists. The character is sometimes described simply as Bayek, but his full commonly used name is Bayek of Siwa.