Ruth is the daughter of the late "lawyer Varnum" and has a little more money and a little more education than the other women in town (Prologue.19). She lives with her mother in the big house that once belonged to her father, and rents one of the rooms to the narrator.
She was also friends with Mattie Silver before the accident and helped take care of Mattie before Zeena decided to bring her back to the Frome house. Ruth is an important character. She is the one person in the story who is present from the beginning to the end; she probably knows more about what went down than everybody besides the three starring players.
Did you notice that she also gets the last words of the novella? You can check out our "What's Up With the Ending?" for the lowdown on that. In the mean time we will just say that her final words leave us with a bad, sad taste in our mouths.