Kuro no Tantei

Kuroba Yo is a sadistic detective who enjoys nothing more than destroying his targets emotionally and psychologically through his deductive reasoning. Toiro Arata is a naive and slightly masochistic boy who gets duped by Yo into becoming his assistant, or slave. These two team up to become the "Black & White" detective, solving crimes and chasing after criminals.

(Source: MangaHelpers)