In Assassin's Creed Odyssey, Kassandra is canonically depicted as the player character.
Odyssey lets players choose between Kassandra and Alexios at the start of the game, and either can be used in the player's personal playthrough. Ubisoft's associated canonical material, including the novelization, identifies Kassandra as the canonical protagonist. In the game, the selected character is a mercenary known as the Eagle Bearer.
The story is set in ancient Greece during the Peloponnesian War, beginning in 431 BCE. Kassandra's journey involves family conflict, the Cult of Kosmos, and the pursuit of the Staff of Hermes Trismegistus. The game also includes dialogue choices, branching outcomes, naval exploration, and large-scale battles.
Alexios is therefore a valid playable choice but not the canonical one. Phoibe is a child companion, while Aspasia is an influential historical character connected to the game's political story. Odyssey is a prequel to many earlier Assassin's Creed settings, although its modern-day storyline links it to the wider series.