Kassandra is the canonical protagonist of Assassin's Creed Odyssey.
Odyssey lets players choose to play as Kassandra or Alexios, both descendants of the Spartan king Leonidas. The choice changes the playable character's presentation, but the game's novelization identifies Kassandra as the canon protagonist. In the game, the selected character becomes a mercenary known as the Eagle Bearer and travels through the Greek world during the Peloponnesian War.
Kassandra's story begins in the fifth century BCE and includes Sparta, Athens, the Aegean islands, and other locations associated with classical Greece. Alexios remains a fully playable option, which is why many players encounter him as their own hero.
This fact is often confused because the game does not force one protagonist during character selection. Canon refers to the version of events used by related official material, especially the novelization. The choice does not mean Alexios is absent from the game; his role changes depending on the player's selection.