After the tragic death of her parents, Heidi is forced to move in with her Uncle, Dr. Palmer Jekyll in New England. It is here that she meets a boy named Aaron Freidus, and he claims that her uncle is a monster. Heidi thinks Aaron's theories are ridiculous. Heidi thinks that Aaron is unintentionally connecting the story of Dr. Jekyll and Mr. Hyde to real life.
When Heidi meets up with her uncle, strange things begin to happen, and she starts to believe Aaron and what all the towns people think about him. The longer that she stays with her uncle, the more she begins to fear him.
Heidi is twelve years old. She has long, straight, brown hair. She also has green eyes.
Early in the novel, Heidi is described as being very sad and depressed as a result of her parents death. However, Heidi makes many attempts to cheer up and put the past behind her.