In Assassin's Creed Odyssey, Kassandra is the canonically playable protagonist in the game's story.
Odyssey lets the player choose between Kassandra and Alexios at the start. The selected character becomes the playable mercenary, while the other sibling takes a different role in the narrative. Ubisoft later identified Kassandra as the canon protagonist for the game's story and related continuity.
The game is set in Ancient Greece during the Peloponnesian War, beginning in 431 BCE. Kassandra can travel across the Greek world, take part in naval exploration, and make decisions that affect relationships and outcomes. The role-playing structure allows substantial variation, but the protagonist's canonical identity is distinct from the player's initial choice.
Alexios is therefore a valid playable choice but not the canon answer. Aspasia and Phoibe are important characters, yet neither is the game's selectable protagonist. Odyssey is also separate from Assassin's Creed Origins, whose central historical character is Bayek of Siwa.