Moko to Yuganda Satsujinki-domo

Summary

22-year-old Haijima Hitsugi has been transferred to the unusual murders section, where he will investigate mysteries with Utagata Moko. Moko is actually the same age as he is, but she looks, dresses, and acts like a little girl. While dealing with Moko is basically like taking care of a child, she is also brilliant at unraveling some pretty twisted murder mysteries. (Source: MangaHelpers)