Kassandra is the canonical protagonist of Assassin's Creed Odyssey.
Odyssey allows players to choose between Kassandra and Alexios at the start of the game. Both choices use the same central family role, but later Assassin's Creed material identifies Kassandra as the canonical version of the protagonist. The game follows this mercenary across the Greek world during the Peloponnesian War.
Kassandra is the granddaughter of King Leonidas I through her family line and carries the Spear of Leonidas, an Isu artifact. Her story includes conflicts involving Sparta, Athens, the Cult of Kosmos, and her own family. The narrative also features dialogue choices and branching outcomes.
Alexios is not an invented distractor: he is the selectable alternative and appears in the game depending on the player's choice. That is why the question asks for the canonical protagonist rather than simply the playable protagonist. Kassandra also appears in later franchise material, including the crossover story in Assassin's Creed Valhalla.