In Assassin's Creed Odyssey, Kassandra is the canonical protagonist.
The game lets players choose Kassandra or her brother, Alexios, as the playable character. Ubisoft later identified Kassandra as the canonical version in the series' official novelization and related canon, while Alexios remains a playable option in the game itself.
Kassandra is a Spartan mercenary whose story unfolds during the Peloponnesian War. Her family history connects her to Leonidas I, the Spartan king whose stand at Thermopylae appears in the game's opening sequence.
Choosing Alexios does not create a separate historical timeline within the game's branching presentation; it changes the performed version of the protagonist. Leonidas and Brasidas are important historical figures, but neither is the player character.