In Assassin's Creed Odyssey, Kassandra is the confirmed canonical protagonist.
Odyssey was released in 2018 and lets players choose between Kassandra and Alexios as the playable mercenary. Ubisoft later identified Kassandra as the canon version of the protagonist, while Alexios serves as the game's version of Deimos, a major opposing character in the main storyline.
The game is set in ancient Greece during the Peloponnesian War, beginning in 431 BCE. Kassandra is a descendant of King Leonidas I of Sparta and becomes involved in the conflict between Athens and Sparta, the Cult of Kosmos and the fate of her family. The adventure includes mainland Greece, the Aegean islands and several other regions.
The choice screen can make the answer seem ambiguous because both characters are playable. Canon, however, refers to the continuity established by official follow-up material, including the novelization, which uses Kassandra. Phoibe and Aspasia are important characters but are not the playable protagonist.