Who is the protagonist players control in Assassin's Creed Origins?

The protagonist players control in Assassin's Creed Origins is Bayek of Siwa.

Bayek is a Medjay, a protector and law-enforcement figure from the Siwa Oasis in ancient Egypt. After a personal tragedy, he and his wife, Aya, investigate a group known as the Order of the Ancients, whose actions help establish the foundations of the later Templar tradition.

Origins is set primarily during the first century BCE, when Egypt is ruled by Cleopatra and caught in a power struggle involving Julius Caesar and Ptolemy XIII. The game portrays the beginnings of the Hidden Ones, the predecessor organization of the Assassins.

Bayek is sometimes reduced to “the first Assassin,” but the game's historical terminology is more precise: he helps found the Hidden Ones, while the formal Assassin Brotherhood develops later in the series' chronology.