Kassandra is the canonical protagonist of Assassin's Creed Odyssey.
Odyssey lets players choose Kassandra or Alexios as the playable character, but Ubisoft's later canon identifies Kassandra as the central historical figure. She is a Spartan misthios who travels through the Greek world during the Peloponnesian War, which began in 431 BCE.
Alexios remains a valid player-selected protagonist within the game's choice system, so the question requires the word “canonical.” In the game's branching presentation, either sibling can be selected, but Kassandra is the version used by the established wider continuity.
Odyssey is set before the main Assassin Brotherhood exists in its later form. It focuses on ancient Greek conflicts, the Cult of Kosmos, and the Isu-related Staff of Hermes. Unlike many entries, the game has no conventional present-day playable protagonist in the same way as earlier installments.