KEYMAN

Keyman is the resident hero of Rockville City, a place inhabited by both humans and animal people. In the years since his debut as a hero, crime has decreased significantly, which does not look all that great for the local police force. But one day, after he catches yet another group of criminals, Keyman is mysteriously murdered.
Detective Alex and his police force are investigating the murder, and one of their major clues is the words "Dr. Necro," which were carved onto Keyman's body after his death. Their case gets even stranger when Alex unexpectedly meets "Dr. Necro", who appears to be a young girl, in a bar.

What monster could be behind the murder of Keyman, and what does the childlike Dr. Necro have to do with it all?

(Source: MangaHelpers)