Kassandra is the canonically designated protagonist of Assassin's Creed Odyssey.
Odyssey lets players choose either Kassandra or Alexios as the playable character. Ubisoft's official narrative designation identifies Kassandra as the canonical protagonist, while Alexios appears as Deimos, the opposing sibling, in that version of the story.
The game is set in ancient Greece during the Peloponnesian War, beginning in 431 BCE. Kassandra is a mercenary connected to the conflict between Athens and Sparta and to the long-running story of the Cult of Kosmos. The game also explores her family history and the First Civilization.
Because the player can select either sibling, many summaries call both options protagonists. That is accurate for gameplay, but it does not change the canonical designation used for the game's established narrative. Kassandra also appears in Assassin's Creed Valhalla's “Those Who Are Treasured” downloadable story.