Kassandra is presented as the canonical historical protagonist of Assassin's Creed Odyssey.
Odyssey allows players to choose between Kassandra and Alexios as the playable character. The game's later canonical material, including the novelization and series continuity, identifies Kassandra as the historical Eagle Bearer. Alexios remains a valid player-selected character within the game's choice structure, which is why the distinction can be confusing.
Kassandra is a Spartan mercenary whose story takes place during the Peloponnesian War. She carries the broken Spear of Leonidas and becomes involved in conflicts involving Sparta, Athens, the Cult of Kosmos, and the First Civilization.
Assassin's Creed Odyssey was released in 2018. Although the title is part of the Assassin's Creed franchise, its ancient Greek setting predates the historical Assassins and Templars shown in many other games. The game's narrative instead explores earlier origins of artifacts, ideologies, and bloodlines that later connect to the wider series.