J.P. Monroe was the son of two wealthy aristocrats, who showed him very little affection. At some point during his early adolescence, Monroe killed both of his parents and inherited their fortune. He then invested this fortune in the Boiler Room Club, a multi-purpose venue incoprorating a nightclub, live music venue, strip joint and restaurant.
A vain and cruel misogynist, Monroe has little problem finding vulnerable women and having his way with them. He has an on-off relationship with Terri, and a brief fling with the slightly ditsy Sandy, whom he makes love to. After rudely ordering her to leave his premises, Sandy inadvertently stumbles across the Pillar Of Souls, whereupon she is devoured by Pinhead.
Appalled and flabberghasted by Pinhead's actions, J.P. makes a pact with Pinhead - he will bring souls to the Pillar for Pinhead to devour, and in return, Pinhead will eventually take J.P. under his wing and show him pleasures he would never otherwiese experience. However, Pinhead betrays J.P. when he unsuccessfully attempts to bring ex-girlfriend Terri to the Pillar. Pinhead rams a piston into J.P.'s head, and turns him into a cenobite.
As his name states, Pistonhead has a piston put through his head which moves back and forth. He has stitches in his head and a few minor cuts on his body.
He brought himself into a lot of trouble when he purchased the statue in which Pinhead was placed. He accidentally brought Pinhead back to life by sticking his hand in the statue and being bitten by a rat at which blood splattered out onto the statue and flowed into the mouth of Pinhead.
JP didn't know Pinhead was alive until he brought a woman called Sandy in his room, slept with her, and tried to kick her out, but then she was skinned and dragged into a pilar of souls by Pinhead after getting to close to the statue. JP thought Pinhead was going to give him a new life, so he agreed to bringing another woman in his room, who was Terri. He tried to drag her to Pinhead, but she hit him with her brass knuckles, and he was pushed towards Pinhead, who pushed a piston into his head, turning him into a cenobite.