In Assassin's Creed Odyssey, the protagonist's sibling who becomes a major antagonist is Deimos. Depending on the player's chosen protagonist, Kassandra or Alexios is the playable character, while the other sibling takes the identity of Deimos.
The story follows a fractured family during the Peloponnesian War in fifth-century BCE Greece. Deimos is manipulated by the Cult of Kosmos and becomes one of its most dangerous agents. The player's choices affect the family's relationships, the fate of several characters, and which ending is reached.
Alexios is therefore not always the answer to this question: Alexios can be the protagonist or Deimos, depending on the player's selection. Kassandra is the canon protagonist in later series material, but Odyssey allows players to choose between the two siblings. Nikolaos is their stepfather in the game's story, while Stentor is a related military character, making both names plausible but incorrect alternatives.