The Detective Is Already Dead, Vol. 3 (manga) - nigozyu - Bog

Due to my penchant for getting dragged into things, I, Kimihiko Kimizuka, an ordinary high-school student, suddenly was asked to protect national idol Yui Saikawa's Three-billion-yen sapphire!? When there's a detective, there's always a case, and so, another case begins for me and Siesta’s heart’s recipient, Nagisa Natsunagi!