Kassandra is the female protagonist canonized for Assassin's Creed Odyssey.
Odyssey allows players to select either Kassandra or Alexios as the playable mercenary, but the official novelization and later series continuity identify Kassandra as the canonical protagonist. She is a descendant of Leonidas I and carries the Spear of Leonidas through the game's ancient Greek storyline.
Kassandra's journey takes place during the Peloponnesian War and involves family conflict, the Cult of Kosmos, and the search for the truth behind her lineage. Voice actor Melissanthi Mahut provides Kassandra's English performance.
Alexios is not an incorrect choice for gameplay: he is fully playable when selected. The distinction is between player choice and canon, which is why Kassandra is the single answer when the question asks who the series treats as the official protagonist.