Kane Starkiller was the Commander of the Guardians of Lettow in approximately 2 BBY. This made him the de facto apprentice of Darth Vader. Starkiller became jealous once Vader attempted to convince Princess Leia Organa to become his secret apprentice once she began to recognize her darker side, as being the Dark Commander of the Lettow put Starkiller in line to be Darth Vader’s apprentice if something were to happen to the Emperor.