Kassandra is the canonical protagonist of Assassin's Creed Odyssey.
Odyssey allows players to choose Kassandra or Alexios as the playable character, but the wider series canon identifies Kassandra as the protagonist. She is a Spartan mercenary who travels through the Greek world during the Peloponnesian War.
The game is set in the fifth century BC, beginning in 431 BC. Its story includes Athens, Sparta, the Aegean islands, and encounters with historical figures such as Pericles, Socrates, and Alcibiades.
Alexios remains an important character: when Kassandra is selected, he serves as Deimos, a major antagonist connected to the Cult of Kosmos. This choice-based presentation is why some summaries list Alexios as the protagonist, but official follow-up material and the novelization establish Kassandra as the canonical hero.