Kassandra is the canonically chosen protagonist of Assassin's Creed Odyssey.
Odyssey lets players select either Kassandra or Alexios as the playable character at the beginning. Ubisoft later identified Kassandra as the canon protagonist, and the official novelization also follows her version of the story. The game itself accommodates either selection for the player's campaign.
Kassandra is a Spartan mercenary and the grandchild of King Leonidas I. Her story is set during the Peloponnesian War in the fifth century BCE, and the player travels through the Greek world, including Athens, Sparta, the Aegean islands, and parts of the wider region.
Alexios is not an invented alternative unrelated to the narrative: he remains a selectable playable character and can portray Deimos in the story depending on the player's choice. That flexibility causes frequent confusion. Phoibe and Aspasia are important characters, but neither is one of the two player-selectable protagonists.