Kassandra is the canonical protagonist of Assassin's Creed Odyssey.
Odyssey lets players choose either Kassandra or her brother Alexios as the playable character. Ubisoft's official narrative establishes Kassandra as the canonical choice, while the selected character becomes the game's opposing Deimos. The story follows the protagonist across the Greek world during the Peloponnesian War in the fifth century BCE.
Kassandra is a descendant of King Leonidas I of Sparta and carries the Spear of Leonidas, an important Isu artifact in the game. Her journey involves family conflict, the Cult of Kosmos, and the search for information about her heritage. The player can make dialogue and story decisions, but the canon identity remains Kassandra.
Alexios is not an incorrect name for the playable role in an individual player's save; he is the alternative player character and the canonical Deimos. That distinction is why Kassandra is the specific answer when the question asks which sibling is canonical rather than simply playable.