Who is Mattie Devore from Bag of Bones and what is their importance?

Mattie Devore lives in a double wide trailer on Wasp Hill Road at Dark Score Lake. At first glance, Mattie could pass for a young girl of twelve or fourteen, but closer inspection reveals that she is more likely closer to nineteen or twenty. When Mike meets her for the first time, she is wearing paisley shorts and a Kmart style smock. She has long yellow hair that is held back by a red elastic and Mike's first impression is 'trailer trash.' She has dark circles under her eyes and a look of terror bordering on exhaustion. Her daughter, aged either three or four had taken it upon herself to go to the lake alone.