Tenshi-tachi no Gogo Special II

Main hero is high school student. But he also steals money and assets from villains to return it to the proper owners. One day he received a request by mail to get "Blue Diamond" out of debt situation.

The only common part with the predecessors is that it's a school mystery. Game series is changing very slowly over time and thus starting to look bleak compared to competitors. Anyway, character design is cute and it's still possible to enjoy the game heartfully.