It's an older manga and it is completed.
Can't remember the name but it kind of reminded me of Ao no Exorcist the only major difference though is that there were no exorcists.
I believe it begins with a kid and his brother, the brother became a cop to catch their parents' killer?
The one brother does his own kind of investigation into some murders and eventually intersects with his cop brother and I believe the cop bro dies while the other brother gains a portion of the killers powers.
Because of the powers he gains the ability to see demons and other supernatural phenomenon.
The surviving brother eventually becomes a cop and investigates all sorts of supernatural stuff.
One case had something to do with a magical pelt that when worn allows the person to become a werewolf.
Another case had something to do with goblins tearing things apart because they weren't being paid their due.
In order for the Goblins to be useful or helpful had to leave out some kind of candy or they would destroy stuff.
Some additional info, it is set in Europe and his power had something to do with a spear. I think it was the Spear of Longinus.
Found it through here... Figures I post something and then find a tagline and remember it was mostly based on Irish mythology. The manga is called AR∀GO. The spear was Brionac not the Spear of Longinus. Here is the summary for it:
The story centers around two twins, Arago and Yuan, whose parents were slaughtered by monsters. Years later, the vengeful Arago crosses paths with Yuan, who has since become a police officer, just as blood begins to flow again in the streets of London.