Showing The Devil's Whisper by Miyuki Miyabe

Slowly, the answers are uncovered by sixteen-year-old Mamoru, the nephew of the taxi driver currently being held by the police on charges of manslaughter for the death of the third victim. Determined to help his uncle, the enterprising young protagonist discovers that the girl killed by his uncle's taxi had participated in a devious scam to separate vulnerable men from their money, and that three of the four girls involved in the ploy are now dead. When a powerful businessman reveals new evidence that could free Mamoru's uncle, Mamoru decides he must go all out if he is to save the last of the four girls being targeted by the real killer. And then the killer contacts him.