Kassandra is the canon Eagle Bearer in Assassin's Creed Odyssey.
Odyssey lets players choose Kassandra or Alexios as the playable character, but the official novelization identifies Kassandra as the canonical protagonist. In the game, the chosen character is a Spartan mercenary known by the title Eagle Bearer and becomes involved in the Peloponnesian War.
The story is set in Greece during the fifth century BCE. Kassandra's journey involves her family, the Cult of Kosmos, the ancient Isu, and historical figures such as Socrates, Pericles, and Aspasia.
Alexios remains a valid playable choice in the game, which is why the answer can be confusing. The question concerns the canon version of the protagonist rather than the character selected by an individual player.