Mattie is Zeena Frome's cousin. Her father had been the envy of relatives; he'd moved to Connecticut, married, and led everyone to believe his business ventures were successful. After his death, the family found out that he had mismanaged the money they had invested in his business ventures. As a result, their money was gone. The shame of this revelation contributed to the death of Mattie's mother soon after the death of her father.
At age 20, Mattie was left alone with only fifty dollars from the sale of her piano. She worked at odd jobs, and it was not long before she became ill. Mattie's relatives punished her for her father's mismanagement of their money by refusing to give her financial assistance. Zeena, however viewed Mattie's impoverishment as an opportunity and invited Mattie to move to Starkfield. Mattie became Zeena's indentured servant, working as Zeena's aide in exchange for a roof over her head and food to eat.
During the year that Mattie lives with the Fromes it is apparent that Mattie is "quick to learn, but forgetful and dreamy." She rarely completes her daily tasks; however, rather than listen to Zeena complain (and because he is in love with Mattie), Ethan steps in and "supplement(s) her unskilled efforts."
Mattie's personality contrasts with that of Zeena. Mattie is not a "fretter;" she doesn't complain. She is healthy, happy, and pretty. She is full of life and enthusiastic. She admires Ethan's observations and thoughts, and shares his appreciation of nature.
Throughout the novel, Wharton foreshadows the tragedy that befalls Mattie. Mattie feels insecure about her position at the Fromes because she senses Zeena's dislike of her. Several times she asks Ethan if Zeena's intention is to send her away. Mattie experiences an internal conflict when Zeena finally does want Mattie out of the house. Mattie doesn't know what she will do. She isn't trained: She has no money, and no connections to help her find employment. When Ethan takes Mattie to the train station (which they never reach), she declares her love for him and admits that she doesn't think she can live without Ethan. Their aborted suicide attempt transforms Mattie forever.
As a result of the accident, Mattie is paralyzed from the neck down. At the time of the Epilogue, she has lived with the Fromes and been cared for by Zeena for over twenty years. Mattie has become querulous, whining, and petulant. The beautiful, sensitive, and loving girl that Ethan fell in love with, in a sense, did die the night of the smash-up.