Kassandra is the protagonist used by Assassin's Creed Odyssey's canonical storyline.
Odyssey allows players to choose between Kassandra and Alexios as the playable character. Both portray the Spartan mercenary known as the Eagle Bearer, but Ubisoft's later canonical material identifies Kassandra as the character who lived through the main historical events.
The game is set during the Peloponnesian War in the 5th century BCE. Kassandra's story begins in Kephallonia and expands across the Aegean world, including mainland Greece and numerous islands. The narrative includes family conflict, political factions, mythical creatures, and the Cult of Kosmos.
Alexios is not an incorrect gameplay choice; he is the alternate player-selected version of the protagonist and can appear in another role depending on that choice. The answer Kassandra refers specifically to the canon established outside the player's selectable option, not to every possible playthrough.