According to Assassin's Creed canon, Kassandra is the protagonist of Assassin's Creed Odyssey.
Odyssey lets players choose between Kassandra and Alexios at the start of the game, and either can be used as the player character during the campaign. Ubisoft's broader canon identifies Kassandra as the historical protagonist, while Alexios appears in the story as Deimos.
Kassandra is a Spartan mercenary who travels through the Greek world during the Peloponnesian War. The game is set in the fifth century BCE and features Athens, Sparta, the Aegean islands, and figures such as Socrates and Pericles.
This question is commonly confused because the game permits both character choices. The choice affects the player's presentation of the story, but official canon and related expanded material treat Kassandra as the central protagonist.