The Detective Is Already Dead, Vol. 1 (manga)

I, Kimihiko Kimizuka, was an ordinary high school student. Sure, I had a penchant for getting dragged into all sorts of trouble, but was otherwise ordinary-until I met her. The detective, Siesta. Her angelically beautiful face and cool disposition in the face of plane hijackers swept me off my feet…and the next thing I knew, I'd spent the next three years off on a whirlwind adventure acting as her assistant. After all, I was the Watson to her Sherlock. But then…she died. It didn't end there, though. No, our story only really began a year later when I met a bratty, snotty high school girl who seemed oddly familiar…

Volume 1 of The Detective Is Already Dead manga series by nigozyu.