Jessica Sorrow was born as a normal human, but legend has it that one day something so terrible, so mind-shatteringly horrible happened to her that she decided to completely stop believing in the world. She retreated deep within her own mind, choosing to completely ignore the reality of the outside world. Her body withered into a pale, emaciated frame and her hair fell out from neglect and indifference and Jessica Sorrow became The Unbeliever. Most of her days as The Unbeliever were spent in a deep sleep as Jessica lay dormant within her own mind, but from time to time she would wake up for some reason or another and venture out into the streets of the nightside. When The Unbeliever walked the streets, even those on the Street of the Gods locked up and hid until she fell dormant once more.
For years she existed as such until one day some part of her mind called out to John Taylor, requesting him to do what he was famous for; find something. The Unbeliever appeared before John in St.Jude church and he presented her with a teddy bear; a lost relic from Jessica's old life. With something firm to hold on to, Jessica Sorrow slowly became more sane, and eventually became one of the new Authorities. She claimed to be getting better when she met with John Taylor along with the new Authorities.
"Jessica Sorrow, the Unbeliever. She gave up her humanity to become what she is. She doesn't believe in anything. And because she doesn't believe with such utter certainty, all the world and everything in it are nothing to her. None of it can affect her in the least. She can go anywhere, and do anything, and she does. She can do terrible, distressing things, and she does, and nothing can touch her. She has no conscience and no morality. No pity and no restraint. The material world is like paper to her, and she rips it apart as she walks through it."
Jessica Sorrow's power stems from her complete and utter indifference to the world, her disbelief is so strong that it forces it's way into reality and makes itself true. At the disbelieve of the Unbeliever, things vanish, having never existed. Nothing has been able to harm her according to legend, because she doesn't believe anything can, nor does she care.
It is possible that Jessica Sorrow could face off against The Walking Man, pitting absolute faith against absolute disbelief, but it never happens, as the Authorities were reluctant to allow it to happen before giving John Taylor a chance to bring down The Walking Man for fear of what would happen when the two powers collided.
After rampaging in the Nightside she found something that was real to her. John Taylor forced his gift to find something that had been Jessica's before she became the Unbeliever. A teddy bear that he found in the Teddy Bear Graveyard.