Kassandra is the canonical hero of Assassin's Creed Odyssey according to the game's novelization. Odyssey lets players choose either Kassandra or Alexios as the playable character, but the companion novel uses Kassandra as its protagonist.
The game is set in ancient Greece during the Peloponnesian War, beginning in 431 BCE. Kassandra is a Spartan mercenary and a descendant of Leonidas I. Her story involves family conflict, the Cult of Kosmos, and the mysterious Isu artifact called the Staff of Hermes Trismegistus.
Alexios remains a valid playable choice in the game, which is why the question can cause confusion. The choice changes the identity of the player character but does not create two separate campaign histories in the same way as a branching anthology. Ubisoft and the novelization established Kassandra as the canonical version.
Odyssey is set centuries before the earliest Assassin Brotherhood stories. It is connected to the wider mythology through the Isu and the later historical-memory framework rather than through a direct depiction of the Brotherhood.