Satsujin wa Tegami ni Notte

Main character is high school student who hears of his friend being fatally hit by train. He has 7 days remaining till school studies start and he intends to find out the truth behind this accident.

The cover is funny, but it's not a light story. Serial murders, attempt suicide, main hero orphan. Most of the game passes in investigating the school and then the university where the murders occurred. The difficulty level is low, so it's possible to bruteforce. Solving this mystery does not really make an impact.