Kassandra is the Assassin's Creed Odyssey protagonist treated as canon in the game's novelization.
Odyssey lets players choose Kassandra or Alexios as the playable protagonist. In the game itself, the choice changes the character seen in the player’s campaign, while the other sibling takes a different major role. Ubisoft's official novelization, written by Gordon Doherty, uses Kassandra as its protagonist, establishing her as the canonical Eagle Bearer for that version of the story.
Kassandra's adventure takes place in ancient Greece during the Peloponnesian War. She is a Spartan mercenary who becomes involved with the Cult of Kosmos, the conflict between Athens and Sparta, and the deeper First Civilization mythology of the series.
This canon point is often confused with gameplay choice. Alexios is fully playable and is not an incorrect selectable character; Kassandra is the canonical protagonist used by the novelization and later franchise references. Phoibe and Myrrine are important supporting characters, not the Eagle Bearer.