Who is Ruth Fisher van den Meer from Confessions of an Ugly Stepsister and what is their importance?

Ruth is Margarethe's elder daughter who is assumed mentally handicapped because she does not speak. She is a large girl and frequently called an ox. Ruth is the first person narrator of the Prologue and the Epilogue. Clara gives her a windmill toy in the beginning of the story and when they become stepsisters, Ruth cares more for Clara than her own mother. She even speaks or sings a few words to Clara on occasion.