In Assassin's Creed Odyssey, Kassandra is the canonically selected protagonist.
Odyssey lets players choose between two siblings, Kassandra and Alexios, at the beginning of the game. Both can be used as the playable mercenary, while the other sibling appears as Deimos, an important character in the story. Ubisoft's official novelization and later series continuity identify Kassandra as the canonical protagonist.
The game is set in Ancient Greece during the Peloponnesian War, beginning in 431 BCE. Kassandra's journey involves family conflict, mercenary work, the Cult of Kosmos, and encounters with major figures from Greek history and philosophy.
Because the game offers a genuine player choice, many players understandably regard either sibling as their personal protagonist. The question asks for the canonical choice in the broader story, not which character an individual player selected. Kassandra is therefore the specific answer, while Alexios remains a valid playable option.