In Assassin's Creed Odyssey, Kassandra is the name used for the canonically female playable protagonist.
The game lets players choose Kassandra or Alexios as the character they control. Ubisoft's later novelization and franchise canon identify Kassandra as the protagonist of the main story, while Alexios serves as the opposing Deimos character in that version.
Kassandra is a Spartan mercenary living during the Peloponnesian War. Her journey travels across the Greek world and involves family revelations, the Cult of Kosmos, and the ancient artifact known as the Staff of Hermes Trismegistus.
This is why both Kassandra and Alexios can be valid player choices in gameplay, but Kassandra is the single canon protagonist requested by franchise continuity. Phoibe and Aspasia are important supporting characters, not alternate protagonist names.