In Assassin's Creed Odyssey, Deimos is the player character's sibling.
The player chooses Alexios or Kassandra as the playable protagonist, and the other sibling becomes Deimos. The story presents Deimos as a powerful and feared figure connected to the Cult of Kosmos. This family relationship is central to the game's main conflict and creates different dramatic possibilities depending on the player's choices.
Brasidas is a Spartan commander, Stentor is connected to the protagonist's family, and Nikolaos is the protagonist's stepfather in the story. None of those characters is the sibling identity. Another source of confusion is that players can choose either Alexios or Kassandra as the playable hero, but Deimos is the name used for the sibling role regardless of that choice.
Odyssey was released in 2018 and is set in ancient Greece during the Peloponnesian War. It includes dialogue choices, multiple possible endings, and an open-world map spanning mainland Greece and the Aegean region.