Shadow Walkers are a strange breed that possess the ability to slip between the planes of reality, becoming immaterial. This allows them to hide in the shadows, either hunting from the concealment of near invisibility or evading enemies. Though they become corporeal to mate or to eat, they spend most of the rest of their lives walking in the shadows. Shadow Walkers are wary creatures, though on dark autumn evenings when the leaves fall to swirl in the chill, rain damp air, they may be seen drifting out from the shadows.

Dragons are highly-intelligent reptilian creatures that—from a human perspective, at least—appear to live forever. Many different varieties of dragon exist, each with their own unique qualities, habitats, and behavior. Adolescence in dragons is usually marked by the growth of a hatchling’s wings, although not all breeds of dragons grow wings and some breeds have other traits that indicate the beginning of maturation. In Galsreim, dragons and humans coexist peacefully.

Theresa Herondale was found by a member of The Dragon Health Department as a hatchling. She had been beaten to a pulp because she wasn't very strong. She was then taken to Lou, a young girl who takes in abused and unwanted dragons, who quickly put her in her care. Theresa was extremely frightened, but quickly found a friend in Will and Jem. When Jem went away to see Mercedes, a girl who works with Lou, Theresa found herself even more scared than before. It took Theresa years to finally get over her fear.

Now she is sweet and docile. She always contributes to working with her new found team and has been happy ever since she came to live with them. If anyone were to ask her about her new home, she'd simply say it's a peaceful living home for the broken.