Mr. Postman's Things

So, there is this boy who is jobless and basically lives in a computer store. He comes across this amazing job while going to that address something feels sketchy. The place is really gloomy and the boss is a child who claims to be a demon who works as a detective. They solve mysteries together (like the child solves them the boy just listens).

Yeah, that pretty much I remember. Plus, the demon detective is completing with another similar detective for a important role in hell or something like that.
If anyone knows the same please let me know!