The mangaka should really have got someone with actual skills to write the story. Without the demon, just making it all work through good writing. MC dude could still have had an accomplice, of course, as it's only realistic, but it wouldn't need to be a demon. Just make it someone older and extremely disgruntled. His advice would allow the MC to avoid pitfalls with the detectives.
Too bad this author couldn't write a tolerable plot even if his life depended on it, by the looks of it.