Kassandra is the Spartan misthios who can be played in Assassin's Creed Odyssey's canon story.
Odyssey lets players choose between Kassandra and Alexios as the playable character. Ubisoft's official canon identifies Kassandra as the protagonist whose story is represented in related material, including the novelization. In the game, the chosen character is a mercenary, or misthios, during the Peloponnesian War.
The adventure is set in ancient Greece in the fifth century BCE. Kassandra's journey involves Sparta, Athens, the Greek islands, and the Cult of Kosmos. The narrative also connects the historical world to the broader Assassin's Creed mythology through Isu artifacts and locations.
Alexios is not an unrelated character: he is the alternate player choice and has an important role depending on the selection. Aspasia and Phoibe are supporting characters. The distinction between player choice and canon protagonist is why Kassandra is the precise answer here.