Kassandra is the canon protagonist in the official novelization of Assassin's Creed Odyssey.
Odyssey lets players choose either Kassandra or Alexios as the playable mercenary descended from Leonidas. Ubisoft's official novelization, written by Gordon Doherty, follows Kassandra, establishing her as the canonical version for that adaptation and the wider continuity associated with it.
The game itself supports both choices, so Alexios is a valid playable character but is not the protagonist used by the novelization. Myrrine is Kassandra's mother, while Aspasia is a major historical and narrative figure in the game.
Odyssey is set in ancient Greece during the Peloponnesian War, beginning in 431 BCE. It predates the formation of the Assassin Brotherhood shown in Origins and instead explores the earlier world of the Isu, the Templars' distant precursors.