Kassandra is the protagonist selected as canon in Assassin's Creed Odyssey.
Odyssey allows players to choose between Kassandra and Alexios as the playable mercenary. Ubisoft's official novelization and later franchise material establish Kassandra as the canonical protagonist, even though both choices are playable within the game itself.
The story is set in ancient Greece during the Peloponnesian War, beginning in 431 BCE. Kassandra is a descendant of King Leonidas I of Sparta and carries the broken Spear of Leonidas, an Isu artifact that functions as a gameplay weapon.
Alexios is not an incorrect description of the game's playable options; he is the alternative character choice. The question concerns the canon established outside the player's individual playthrough. Odyssey was released in 2018 and includes Athens, Sparta, the Aegean islands, and other Greek locations. Its branching dialogue and choices make individual stories vary, but the canonical identity of the protagonist is Kassandra.