Elise Reiner: Who is she?
Elise Reiner: A parasite. She befriended your astral body and lured you into the further just as Dalton has been.
Tucker: A picture's worth a thousand words.
Specs: Words are worth a thousand words, too.
Elise Reiner: A sound man, by not advancing himself, stays the further ahead of himself.
Elise Reiner: It's not the house that is haunted. It's your son.
Foster Lambert: Dalton scares me when he gets up and walks around at night.
Answer: It's implied that the girl went crazy and killed her family, but it's not really elaborated on. They're just some of the many spirits to be trapped in and haunt the Further, likely due to the violent nature of their deaths.