Goth

A rather morbid, sombre manga about two high school students, Kamiyama Itsuki and Morino Yoru, who have a peculiar interest in deaths and especially in murders that are committed near to them. Due to their strong fascination with murder and highly-developed analytical skills (along with some luck), they manage to solve the murders that have even eluded the authorities. However, acting on their own sense of justice and in self-interest, the murderers are often able to escape with the murders that they have committed. Often, it is Kamiyama that solves the cases even to the most subtle of details, but he is only interested in one thing, and that is the method by which Yoru should be killed.

Note - Regardless of appearances, the fact that this was serialized in Shounen Ace makes Goth a shounen manga.