John Strange is a former priest who, in the back history of the story encountered a demon and was thrown out of the clergy and has set up privately to be revenged, essentially.
His purpose is to hunt down demons on earth and eradicate them, in a bid to revenge himself upon this demon.
So he's quite a gentle guy. A wounded soul, I think, is the best description. He's got quite a lot of physical and emotional darkness about him.