Levi and Cebastian are half demons. Levi is a fairy prince who doesn't want to be treated as such. He moved to the human world and refused to tell anyone. He especially left out the fact that he is betrothed to a half demon from a foreign kingdom. Even if he had told Cebastian about his fiancee it wouldn't have helped. Cebastian has developed a massive crush on the one person who doesn't treat him like an outcast.