Kassandra is the protagonist canonically chosen by Assassin's Creed Odyssey's official novelization.
Odyssey lets players select either Kassandra or Alexios as the playable character. The selected character becomes the Eagle Bearer, while the other sibling takes a different role in the story. Ubisoft's official narrative material, including the novelization, establishes Kassandra as the canonical protagonist.
This does not mean Alexios is an invalid playable choice. Both characters can be selected in the game, and the broad events of the campaign adapt to that choice. Deimos is the name used for the sibling's opposing role, while Nikolaos is their adoptive father. Odyssey is set in ancient Greece during the Peloponnesian War, centuries before the events of the earliest Assassin's Creed game.