Who is Mattie Johnson from Hattie Big Sky and what is their importance?

Mattie Johnson is Perilee's second oldest child. She is six years old and talks a mile a minute, but is a pure and kind heart who touches the lives of all around her. Mattie has a doll named Mulie that she treats as though it were a child of her own. She never goes anywhere without it. She is a bit of a talker, and at first a bit overwhelming to Hattie, but soon worms her way into Hattie's heart as she does all who know her. To those close to her, she is their little Magpie.