[@shagranoz] I don't have a 100% solid application yet, but the overall idea is that this particular Demon was born from the Spiritstone of a fallen magical girl. The fallen magical girl was actually a classmate to one of the teachers at Heartshine High (maybe the headmistress), but she couldn't purify the Spiritstone in time and it was corrupted. The demon that formed was able to survive for over 20 years, consuming other demons and gaining power until it become more than just negative emotions given form, but could think for itself. Because of the fallen girl's friendship (or maybe something more) with the girl that became a teacher, the demon feels drawn to her.

There are two ways I could go with this:
First is the bad-guy-turns-good/Mayadere route, in which his bond with the teacher has a positive influence and he fights against the demons with the Magical girls

Second is that he stays bad, and his bond with the teacher makes him possessive of her, so he leads the demons so that he can capture and corrupt her.

Thoughts?