Kassandra is the canonically preferred protagonist of Assassin's Creed Odyssey.
Odyssey allows the player to choose between Kassandra and Alexios as the main character. Ubisoft's officially established canon identifies Kassandra as the protagonist, while Alexios appears in the story as Deimos, a major opposing character. The choice changes the player's presentation of the journey but not the game's broad historical setting.
The story is set in Greece during the Peloponnesian War, beginning in 431 BCE. Kassandra is a mercenary and a descendant of King Leonidas I of Sparta. The campaign includes Athens, Sparta, the Aegean islands, and other parts of the Greek world.
This canon distinction is easy to miss because both selectable characters can lead the player through the main campaign. The novelization of Odyssey also uses Kassandra as its central character. Odyssey is set centuries before Assassin's Creed Origins and does not portray the later Assassin Brotherhood directly.