The protagonist commonly identified as Assassin's Creed Odyssey's canon hero is Kassandra.
Odyssey allows players to choose Kassandra or Alexios at the beginning, with the other sibling becoming a major character in the story. Ubisoft later established Kassandra as the canon protagonist in the official narrative surrounding the game.
Kassandra is a Spartan mercenary descended from the legendary Spartan king Leonidas. Her journey spans Greece during the Peloponnesian War and connects family history, political conflict, and the ancient Isu artifacts that recur throughout the franchise.
This question is distinct from asking for the player-selected character: both Kassandra and Alexios are playable. The single canonical answer is Kassandra because later franchise material, including the novelization, uses her as the protagonist. Odyssey's branching dialogue and choices do not change that official designation.