Kassandra is the canonically selected playable protagonist of Assassin's Creed Odyssey.
Odyssey allows players to choose between Kassandra and Alexios as the mercenary protagonist. Ubisoft's official story materials and later series canon identify Kassandra as the canonical choice, while Alexios appears in another major role depending on the player's selection.
The game is set in ancient Greece during the Peloponnesian War, beginning in 431 BCE. Kassandra is a descendant of King Leonidas I and becomes involved in the conflict between Athens and Sparta, as well as the Cult of Kosmos and the wider Isu-related mythology.
Because both characters can be selected in gameplay, references sometimes treat Kassandra and Alexios as interchangeable player avatars. The question asks for the canonically selected protagonist, rather than every playable option.