In Assassin's Creed Odyssey, Kassandra is the canon protagonist in the game's modern lore. Players may choose either Kassandra or Alexios as the playable character, but official series material identifies Kassandra as the canonical Eagle Bearer.
Odyssey is set in ancient Greece during the Peloponnesian War, beginning in 431 BCE. The player character is a descendant of Leonidas I of Sparta and carries the broken Spear of Leonidas, an Isu artifact that grants enhanced abilities.
Alexios remains a valid player-selected version of the protagonist within the game, which is why the question can cause confusion. Kassandra's canonical status is established through later series material, including Assassin's Creed Valhalla, where her long life and continued story are referenced.