In Assassin's Creed Odyssey, Kassandra is the playable character canonically identified as the Eagle Bearer. The game allows players to choose Kassandra or Alexios, but the official modern-day and novel continuity identifies Kassandra as the canon protagonist.
Kassandra is a Spartan mercenary who journeys across the Greek world during the Peloponnesian War. She carries the broken Spear of Leonidas, an ancient weapon connected to her family, and travels with the eagle Ikaros. Her investigations involve family conflict, the Cult of Kosmos, and the wider legacy of the Isu.
Alexios remains a valid player-selected character in the game, which is why this fact can cause confusion. The distinction concerns canon rather than the available gameplay choice. Odyssey is set in the fifth century BCE, long before the historical periods of Origins, Assassin's Creed II, or the original Assassin's Creed.