BK, Bokurano Daibouken 3

As hero and mayor it is your task to protect your town, which by now has evolved into a large city and has set up a crystal mining operation.
This becomes difficult when an attack of monsters at a school turns out to be the front for powerful enemies who demand that you give up all your crystal to them.
You set out on a journey to defeat them, however the risks will prove dangerous not just to you but to your friends, and what seems a simple mission will rapidly become so much more.