He belonged to one.
The wrong person walked into the wrong place at the wrong moment. What happened on that night, as a result, he lost his life. Or rather, what little of a life he could have had at that point. It started with the wrongful footsteps of a homeless kid into the back of an abandoned theatre with the intention of finding refuge. Jason was nineteen, and had burned any of the bridges he had crossed over the past few years. When the night had ended in heroin withdrawals among other life-shaking events, when things couldn't have gotten any further down, he was soon to be proven wrong.
Along came Ophelia, the restless spirit that claimed the theatre. Since she first made her presence known, the ghost put a chill in the young man's bones. The longer he remained in the theatre after his first terrible night, the more attached the spectral woman became. Attachment soon grew to a level of obsession, causing aggravation in return from Jason. It was a game of tolerance... up until the last of his fuse was lit.
Eventually, agitating the spirit in the decrepit building caused something to snap within her. The act would result in a curse that seeped into his being and put his life on hold.
What was once flesh withered to the scarred bone. What were once piercing blue eyes became densely hollow sockets that showed not an ounce of reflection if a light was shown into them. What were once teeth were turned into fangs. A loss of humanity was in place, as something far more demonic to have altered his being from that point on.
It's hard to say what he is.
To say that he is a demon wouldn't be truthful in its entirety, but... its a start.
In a way, he lost everything, yet he was something far stronger than his former self. He lost a life; what he gained was so much more.