Kassandra is the canonical protagonist of Assassin's Creed Odyssey's official story.
Odyssey lets players choose Kassandra or Alexios as the playable mercenary, but Ubisoft has identified Kassandra as the canon character. The game's novelization also follows Kassandra, reinforcing that choice in the wider official narrative.
The story is set in Ancient Greece during the Peloponnesian War. Kassandra is a descendant of King Leonidas I and becomes involved in conflicts involving Athens, Sparta, the Cult of Kosmos, and the First Civilization.
Alexios remains a valid playable choice within the game, which is why the question can cause confusion. The distinction is not about whether Alexios can be selected; it concerns which version represents the canonical continuity used by later material.