In Assassin's Creed Odyssey, Kassandra's sibling is named Alexios.
Odyssey allows the player to choose Kassandra or Alexios as the protagonist. The other sibling then occupies the opposing role of Deimos within the story, so Alexios is Kassandra's sibling when Kassandra is selected as the playable character. The narrative centers on the divided Spartan family and the player's attempts to uncover what happened to it.
The game is set in 431 BCE, during the Peloponnesian War between Athens and Sparta. Kassandra or Alexios is a misthios, or mercenary, who travels through the Aegean world and encounters historical figures, cultists, and mythological elements. Leonidas is their ancestor, not their sibling, while Nikolaos is their adoptive father in the family story.
The name Deimos can cause confusion because it is the title used by the sibling in the antagonist role. The character's personal name depends on the player's choice, but the canonical female protagonist is Kassandra and her sibling is Alexios.