Kassandra is the canon protagonist of Assassin's Creed Odyssey's novelization.
Odyssey lets players choose either Kassandra or Alexios as the playable character. The game's branching presentation supports both choices, but the official novelization by Gordon Doherty uses Kassandra as the protagonist, establishing her as the canon version for the wider story.
Kassandra is a Spartan misthios who travels across the Greek world during the Peloponnesian War. The game is set in the fifth century BCE, long before the historical Assassin Brotherhood is formed. Its plot instead connects to the Isu, the First Civilization, and the ancient origins of several later conflicts.
Alexios is therefore a valid player character but not the canon choice used by the novelization. Odyssey was released in 2018 and follows Assassin's Creed Origins chronologically in the series' ancient-world period, while its modern-day sections continue Layla Hassan's storyline.