Kassandra is the canon protagonist of Assassin's Creed Odyssey according to Ubisoft's established story.
Odyssey lets players choose between Kassandra and Alexios at the start of the game. The selected character becomes the playable Spartan mercenary, but later Assassin's Creed material identifies Kassandra as the canonical protagonist. Alexios instead appears in the story in the role of Deimos, the opposing sibling.
The game is set in ancient Greece during the Peloponnesian War, beginning in 431 BCE. Kassandra's journey involves her family, the Cult of Kosmos, Spartan and Athenian politics, and myth-inspired locations and creatures. She is also connected to the Isu-related Staff of Hermes Trismegistus in the modern-day storyline.
Players sometimes answer Alexios because both choices are playable and the game presents them as equivalent options at first. The distinction is specifically about canon, not about whether Alexios can be selected in gameplay. Odyssey was released in 2018.