Kassandra is the canon protagonist of Assassin's Creed Odyssey according to the game's novelization.
Odyssey lets players choose between Kassandra and Alexios as the playable Spartan mercenary. In the game itself, either character can be selected, and the chosen hero becomes the center of the player's version of the story. Ubisoft's associated novelization, however, uses Kassandra as its protagonist, establishing her as the canonical choice for the wider continuity.
Kassandra's story unfolds in Greece during the 5th century BCE, around the Peloponnesian War. She is connected to the Spartan royal family and becomes involved in a search for her relatives while confronting the Cult of Kosmos.
Alexios is therefore a valid playable choice, but not the canon protagonist identified by the novelization. Myrrine is Kassandra's mother, while Aspasia is a historical character who appears in the game's political storyline.