Bayek of Siwa is the Medjay who becomes the main protagonist of Assassin's Creed Origins.
Origins, released in 2017, is set in Ptolemaic Egypt during the first century BCE. Bayek is a Medjay, a protector and law-enforcement figure whose responsibilities connect him closely to local communities. His personal loss drives his investigation into a secret organization threatening Egypt.
The game presents Bayek as one of the founders of the Hidden Ones, the organization that later develops into the Assassin Brotherhood. His wife, Aya, is also a major character and an important participant in the political and historical story.
Aya of Alexandria is therefore a close but incorrect alternative: she is playable in certain sections and central to the plot, but Bayek is the principal protagonist. Senu is Bayek's eagle companion, while Kassandra belongs to Assassin's Creed Odyssey and is not a character from Origins.