The protagonist who helps found the Hidden Ones in Assassin's Creed Origins is Bayek of Siwa.
Bayek is a Medjay, a traditional protector and lawkeeper in ancient Egypt. The game follows his investigation into the forces responsible for the deaths of people close to him, gradually revealing a conspiracy involving the Order of the Ancients.
Bayek and Aya establish the Hidden Ones, the organization that later develops into the Assassin Brotherhood. Origins therefore depicts the early formation of the creed rather than a mature Assassin order with the familiar medieval structure.
The game is set in Ptolemaic Egypt during the first century BCE. Aya, Bayek's wife, is also a crucial historical figure in the story, but she is not the protagonist named in the question. Kassandra belongs to Assassin's Creed Odyssey, while Basim is the central character of Assassin's Creed Mirage.