Makai Tantei Meiousei O

Say, did you think that humans were the one standing at the top of the food chain? That's a great misunderstanding. The ones who are located at the top of hierarchy of this world are "Them" who live inside the darkness. By no means do they come to the surface world, the one we humans live. What we understand about "Them" is only that they absolutely hate having their existence to become known to human society. While there are some of "Them" with a form resembling that of a human being, there are others that look like a completely different kind of living creature. It is rare, but sometimes trouble between humans and "Them" can happen. Humans who discover the secret of "Their" existence, a stray "Them" preying on humans, and many other problems that I can not talk about to anyone. In those cases, in order to resolve those incidents involving "them", an utility man is employed. That man's name is "Pluto O"... that's me. What I told to you only was what I know. If you ever get in troubles, call my name three times. Say "Pluto O! Pluto O! Pluto O!", and if you're lucky I'll show up to help. All the volumes of this series were released under the name of Echizen Matarou (越前魔太郎); the true identities of the writers were later revealed on Kodansha's page. Source: Kodansha Book Club