Kassandra is the canonical playable protagonist in Assassin's Creed Odyssey.
Odyssey allows players to choose either Kassandra or Alexios as the main character, but Ubisoft's later canon identifies Kassandra as the protagonist. The game is set in ancient Greece during the Peloponnesian War, and the player portrays a mercenary descended from the Spartan king Leonidas I.
Alexios remains a valid player-selected character within the game, which is why the question can be confusing. The choice changes the on-screen identity of the protagonist, while the broader story structure remains largely the same. Kassandra's canonical status is reinforced by later Assassin's Creed material, including her appearance in the crossover expansion “Those Who Came Before” in Assassin's Creed Valhalla.