Who is Doc from Haunted: A Tale of the Mediator and what is their importance?

Doc is the nickname for Susannah's youngest stepbrother. His real name is David. He is 12 years old, but he is a genius. It turns out that the woman ghost talking about "Red" is referring to Doc, who has red hair. He feels guilty for his father having to pull the life support off his mother after she fell into a coma from her breast cancer.