Professor Whimples is a para-scientist who has invented a machine that lets him channel the energy of evil spirits to fight. He's a bit of a hermit and doesn't like to leave his lab, nature and people scare him too much. So he tested his invention on himself. The device worked and once he'd allowed an evil spirit to posess his body he could channel the spirit's energy. However instead of weaponizing the spirit he accidentally made friends with it. And now the evil spirit takes over whenever whimples gets too stressed out to protect him. Or... when he needs to lift something heavy. Or... when there's a spider on the wall. Only in emergencies really.
The only big downside to their artificial symbiosis is that whimples needs a steady supply of precious gems that function as conductors in his machine. But they keep breaking. He just hasn't found the right ones yet.