In apperance, Jennifer is 19 years old, extremely timid and has only one friend in the world: an old dog named Brown. In truth, Jennifer is a young girl with a pretty f'd up life. When she was young, her parents died in a zeppelin crash. After that, she was taken in by Gregory Wilson. Gregory had recently lost his son, Joshua, and was more than willing to take in another child, given that they wear Joshua's clothes, go by Joshua's name and generally replace him in Gregory's life. Gregory's sanity was lacking in other ways, as well, he'd drink and constantly contemplate suicide. He'd lock Jennifer into her basement room for long periods of time, greeting her when his depressive fits were over with one of his "storybooks." The stories were all demented fairy tales, often centering around "Stray Dog." Stray Dog, the evil creature who would eat small children and tempt them with candy.
Eventually, Jennifer was "rescued" by Wendy, a young girl who loved "Joshua." The 2 had been in contact for a long time via letters, and Jennifer moved into the orphanage near Gregory's house (where Wendy lived). There, she ended up engaged in a huge game of make believe where she and her toy bear were a prince, and Wendy was a princess and the other children followed below them in a complex hierarchy known as the "Aristocrats of the Red Crayon." It also seems that Jennifer was a bit of a tyrant herself, with some evidence that the orphanage director found in her a child he despised, even though he'd never known anything but love for one. Possibly not canon, but there are some hints towards is, such as his forgetting her name in roll calls and such and his considering running away from his position after her arrival). She would also scare the other children with stories of Stray Dog, and wolf-cry moments of running around screaming that Stray Dog was coming to gobble them all up. Eventually, however, Jennifer found a small puppy and named it Brown. She kept it hidden from Mr. Hoffman (the owner/director of the Rose Garden Orphanage) in an old gardening shed and the puppy soon became her new best friend. Wendy, however, could not stand the possibility of being replaced. Using her position as "Princess of the Red Rose" she ordered Jennifer to choose between her life, or the life of her best friend. She then had the other members of the aristocracy beat Brown to death. Jennifer reacted to the event with the expected rage, throwing Wendy to the ground and beating her before rejecting the entire group and game. Refusing to accept Jennifer's anger and the end of their friendship, Wendy disguised herself as "Joshua" and returned to Gregory's house. Finding the man in a drunken rage, she lured him back to the orphanage. It's hard to say exactly what happened, but it would seem that Jennifer saw Gregory approaching the orphanage and tried to warn the others of the danger. Not believing another "Stray Dog" story, the children all went to greet Wendy/ask where she had gone. There, in the front yard, Gregory killed all of them. Mr. Hoffman had either finally fled or was also killed.
Jennifer pretty promptly lost her mind, and became fixated on the notion of protecting Brown, and ended up lost in her own bizarre memories of the Aristocrats as twisted and evil. The orphanage became the zeppelin of her imagination, and she was completely lost within it. She finds herself hunted by terrible imps (it only becomes more and more clear that the imps are more her memories of other children than they are the nameless terrors of fairy tales)
By finally coming to terms with the reality of what happened at the orphanage, Jennifer is able to save Brown (at least within herself), and changes her most-important promise from a declaration of love to Wendy to a vow of protection to Brown. Aw.