In Assassin's Creed Origins, the Medjay who founded the Hidden Ones is Bayek of Siwa.
Bayek is a protector of the Siwa Oasis in Ptolemaic Egypt and the main playable character in Assassin's Creed Origins. After a personal tragedy, he and Aya investigate a secret group whose members are known as the Order of the Ancients. Their campaign eventually leads to the creation of the Hidden Ones, the organization that later develops into the Assassin Brotherhood.
Bayek is therefore not formally called an Assassin throughout most of the game. The title reflects the later evolution of his organization, while Bayek himself is identified primarily as a Medjay. Aya, his wife, is also central to the foundation of the Hidden Ones and should not be mistaken for the game's sole protagonist.
Origins serves as a historical beginning for the series' fictional Assassin tradition, set centuries before the medieval events of the first game.