Kassandra is the canon protagonist of Assassin's Creed Odyssey.
Odyssey lets the player choose to play as either Kassandra or Alexios, but Ubisoft's later established canon identifies Kassandra as the protagonist. The selected character is a Spartan misthios who travels through the Greek world during the Peloponnesian War. Kassandra's story involves family conflict, ancient artifacts, and the Cult of Kosmos.
Alexios remains a fully playable choice, which is why both names are often given as answers. In the game's narrative, however, the other sibling takes the role of Deimos, a major antagonist connected to the Cult. The choice affects the player's presentation of the hero but does not change the canon identity used in later series material.
Odyssey was released in 2018 and is set in the fifth century BCE. It is not a direct story about the historical Assassins, whose fictional Brotherhood was not yet established in this period.