Who is Demon from All About Love and what is their importance?

Demon, Lucifer's cousin, comes to visit, along with his wife Felicity (Flick), and brings a message regarding Luicious Appleby. It is another man's suspicion that Luicious has killed before. While he is visiting, he and Flick spend their time helping to solve the murder. They also provide a diversion from the more mundane tasks that lay ahead for Lucifer and Phyllida.