Kassandra is the protagonist identified as canon in the official novelization of Assassin’s Creed Odyssey.
Odyssey lets players choose between Kassandra and Alexios as the playable descendant of Spartan king Leonidas. The game supports both choices, but the 2020 novelization by Gordon Doherty follows Kassandra, establishing her as the canonical protagonist for the broader narrative.
Kassandra’s story takes place in ancient Greece during the Peloponnesian War. She becomes a misthios, or mercenary, and investigates family conflicts, the Cult of Kosmos, and the legacy of the Isu.
Alexios remains a valid playable choice in the game, which is why the question can cause confusion. The canonical choice concerns the protagonist’s identity, not the historical identity of the fictional character’s family members or the game’s player-controlled alternative.