Kassandra is the canonically recognized protagonist of Assassin's Creed Odyssey.
Odyssey lets players choose Kassandra or her brother Alexios as the playable character. The game's story accommodates either selection, but related canonical material, including the novelization, identifies Kassandra as the protagonist. She is a Spartan mercenary who travels through the Greek world during the Peloponnesian War.
Kassandra's story includes family conflict, the Cult of Kosmos, ancient artifacts, and encounters with historical figures such as Pericles and Socrates. The game is set centuries before the earliest Assassin Brotherhood depicted elsewhere in the series, so its central conflict is connected to the wider mythology rather than to a formally established Assassin order.
Alexios is therefore a plausible but incorrect answer to a question asking for the canonical protagonist. Phoibe and Myrrine are important characters, but neither is the player character in the canonical version of Odyssey.