Kassandra is established as the canon playable character of Assassin's Creed Odyssey in the novelization.
Odyssey allows the player to choose between Kassandra and Alexios as the main character. In the game's branching presentation, either can be selected, but the official novelization by Gordon Doherty uses Kassandra as the protagonist. Ubisoft has consequently treated Kassandra as the canonical character for the main Odyssey storyline.
The game is set in 431 BCE during the Peloponnesian War between Athens and Sparta. Kassandra is a Spartan mercenary who travels through the Greek world while uncovering family secrets and confronting the Cult of Kosmos. The story occurs centuries before the events of Origins and does not center on the later Assassin Brotherhood.
Alexios remains a valid player-selected hero in the game, which explains why the question can cause confusion. The distinction is between player choice and the canon used by the novelization and subsequent series material, not between two separate games.