In Assassin's Creed Odyssey, the playable protagonist's sibling is known as Deimos.
The player chooses either Kassandra or Alexios as the main character. The unchosen sibling takes the identity of Deimos and becomes a major figure in the story, connected to the Cult of Kosmos and the protagonist's fractured family history.
This choice changes the protagonist's presentation, but it does not create two different siblings. Kassandra and Alexios are alternative player characters, and the other becomes Deimos in that playthrough's narrative.
Several names can cause confusion. Nikolaos is the Spartan warrior associated with the family, Stentor is another important Spartan character, and Elpenor is a separate antagonist. Deimos specifically identifies the sibling shaped by the Cult's influence.