In Assassin's Creed Odyssey, Kassandra is the canon protagonist according to the game's official story.
Odyssey lets players choose between Kassandra and Alexios as the playable mercenary at the beginning of the campaign. Both characters can fill the protagonist role in the player's version of events, but Ubisoft identifies Kassandra as the canonical Eagle Bearer in the broader series continuity. Her story takes place during the Peloponnesian War in the fifth century BCE.
Kassandra is a descendant of King Leonidas I and carries the Spear of Leonidas, an artifact that connects the personal story to ancient Spartan history. The game presents a large open-world version of the Aegean, including Athens, Sparta, and numerous islands. It also emphasizes dialogue choices, naval travel, and role-playing systems.
Alexios is a plausible answer because he is not merely a background character: he is the selectable alternative protagonist. Leonidas is also important, but he appears as Kassandra's ancestor rather than the player character. The canonical choice matters mainly for continuity in later material, while either choice remains playable in Odyssey itself.