Kassandra is established as canon in the Assassin's Creed Odyssey novelization. The 2018 game lets players choose Kassandra or Alexios as the playable character, but the tie-in novel by Gordon Doherty uses Kassandra as its protagonist.
Odyssey is set in ancient Greece during the Peloponnesian War, beginning in 431 BCE. Kassandra is a Spartan mercenary and a descendant of Leonidas I, whose story connects family conflict, Greek politics, and the wider Assassin's Creed mythology.
The game's choice system allows players to experience different dialogue and decisions. Alexios remains an important character, serving as Deimos in the canonical version of the story, but he is not the protagonist chosen for the novelization.
This distinction can cause confusion because both characters can be selected in the game. The canon designation here comes from the official novelization and related franchise continuity, not from the player's personal choice in an individual playthrough.