Kassandra is the canonical protagonist of Assassin’s Creed Odyssey according to the tie-in novel. The game lets players choose either Kassandra or Alexios as the playable character, but the official novelization uses Kassandra as the central hero.
Odyssey is set in ancient Greece during the Peloponnesian War, beginning in 431 BCE. Kassandra is a Spartan mercenary and descendant of the Spartan king Leonidas I. Her journey takes her across the Aegean world while she investigates her family’s history and confronts the Cult of Kosmos.
Alexios remains an important character in the game, but when Kassandra is selected, Alexios takes the role of Deimos. When Alexios is selected, the roles reverse. This flexible structure can make the canonical choice confusing for players who only experienced one version.
The novel Assassin’s Creed Odyssey: The Official Novelization, written by Gordon Doherty, establishes Kassandra as the protagonist for that narrative continuity. The answer is therefore not based simply on which character an individual player chose.