Kassandra is the canon protagonist of Assassin's Creed Odyssey according to the game's novelization.
Odyssey lets players choose between two siblings, Kassandra and Alexios, as the playable character. The game accommodates that choice in its presentation, but the official novelization uses Kassandra as the central character. This establishes Kassandra as the canonical protagonist for continuity outside a player's individual selection.
The story takes place in ancient Greece during the Peloponnesian War, beginning in 431 BCE. Kassandra is portrayed as a misthios, or mercenary, who becomes involved in family conflict, political struggles, and the wider legacy of the Isu and the First Civilization.
Alexios remains a major character and can be selected as the player's hero in the game, which is why the two names are often confused. Phoibe is Kassandra's young friend, while Aspasia is an Athenian political figure. Neither is an alternative protagonist choice. The canon designation comes from the novelization rather than from a restriction preventing players from choosing Alexios in the game.