Kassandra is the canonical protagonist established for Assassin's Creed Odyssey.
Odyssey allows players to choose Kassandra or Alexios as the playable character, but the game's wider canon identifies Kassandra as the historical protagonist. She is a Spartan mercenary who becomes involved in the political and family conflicts of fifth-century BCE Greece.
The adventure takes place during the Peloponnesian War, which began in 431 BCE. Kassandra can travel across mainland Greece and the Aegean world, meeting figures such as Socrates, Pericles, and Aspasia.
Alexios is not an unrelated character: he is available as the alternate player choice and appears in the story in a different role depending on the selection. This choice system is why many players remember Alexios as the protagonist, even though Kassandra is the canonical version in later series continuity.