Kassandra is the canon protagonist of Assassin's Creed Odyssey according to the game's tie-in novel.
Odyssey lets players choose between Kassandra and her brother Alexios as the playable character. The game's central role is therefore selectable, but the official novelization, Assassin's Creed Odyssey, follows Kassandra and establishes her as the canonical protagonist for the shared story.
Kassandra is a Spartan mercenary living during the Peloponnesian War. Her journey crosses the Aegean world and involves family conflict, the Cult of Kosmos, and the legacy of the First Civilization. The game was released in 2018.
Alexios is not an incorrect gameplay choice: he can be selected as the player character. The distinction is that Kassandra is the version used by the novel and subsequent canonical references.