Kassandra is the canon protagonist of Assassin's Creed Odyssey according to the game's novelization.
Odyssey lets players choose to play as either Kassandra or her brother Alexios. In the game itself, that choice changes the playable character, so both names are valid player options. The official tie-in novel, however, uses Kassandra as its protagonist, establishing her as the canonical choice for the story's wider continuity.
Kassandra is a Spartan misthios, or mercenary, living during the Peloponnesian War. Her journey takes her across the Aegean world and involves the Cult of Kosmos, her family, and the ancient civilization that predates the historical Assassin Brotherhood.
The game is set centuries before Origins, the title that depicts the creation of the Hidden Ones. Odyssey therefore does not feature the formal Assassin Brotherhood. Its connection to the franchise comes through the wider struggle surrounding the Isu, the Cult of Kosmos, and the ideological roots of the series.