Kassandra is the canonical protagonist of Assassin's Creed Odyssey.
Odyssey lets players choose between Kassandra and Alexios as the playable misthios, or mercenary. The game's narrative can be played with either character, but official series material identifies Kassandra as the canon protagonist. Alexios instead takes the role of Deimos in the canonical storyline.
The story is set in ancient Greece during the Peloponnesian War, beginning in 431 BCE. Kassandra's journey involves her family, the Cult of Kosmos, and the search for the truth behind her ancestry and the ancient world.
The choice system is why this fact can be confusing. Unlike a game with only one selectable hero, Odyssey presents two options for the player character. Canonical status is therefore different from simple playability: both characters can be controlled, but Kassandra is the established protagonist in the wider continuity.