Once there was a man in the Nightside who was seeking the true love, but he couldn't find it. But then the Devil, the lord of the hell, showed up to offer him a deal, a deal which guaranteed the man would meet his true love in a certain place at the certain time, but after ten years he had to pay. He met the woman, fell in love with her and married her. For ten years the woman was his true love. But after that time the Devil came once again and demanded his payment, his soul. The man voluntary followed the Devil to hell, there he was tortured and showed the true form of his wife, a succubus, but he never stopped loving and after years of torture and never stopping love, the Devil send him out of the hell. However making a deal with the Devil, the man never ever could go to heaven. So he landed back on earth, unable to go to hell nor heaven and became the man called the Sinner.
The woman he married was Pretty Poison the Succubus send from the hell, after the Sinner was sent back she followed him to learn to understand something called the true love.
The Sinner often goes to the library to read books to find out more about Hell and Heaven, but the real reason why he reads all the time is because don't want to think about his current situation.
The Sinner can't be killed by any weapon or magic, further more he can reflect magic to its user or cancel it. He doesn't age and heals any wound he receives.
In addition Pretty Poison protects him, as if he would need protection.