In Assassin's Creed Odyssey, the player character's older sibling is known as Deimos. Deimos is the name used after the sibling is taken by the Cult of Kosmos and shaped into a powerful enforcer.
Odyssey lets the player choose either Alexios or Kassandra as the playable character. Whichever sibling the player selects, the other becomes Deimos. This makes Deimos a canonical story identity rather than a permanently fixed playable gender or appearance. The sibling relationship is central to the family's tragedy and to the player's effort to confront the Cult.
The game is set during the Peloponnesian War in the fifth century BCE. The family includes Myrrine, Nikolaos, and the legendary Spartan king Leonidas. The precise outcome of several relationships can vary because Odyssey includes dialogue choices and branching decisions, but the name Deimos remains the designation attached to the sibling raised by the Cult.