Before the events of the story, Duke was an important figure in the Great War, in which humans and the Entelexeia fought. He fought alongside the Entelexeia Elucifer so that humans and the creatures could live together in peace. Although they were victorious, some humans no longer trusted the Entelexeia and executed Elucifer. After this betrayal, Duke lost all faith in humanity and cut all ties with them. He only appeared when he felt it was necessary for the protection of the world and the Entelexeia.