Who is Roberta Teys from A Great Deliverance and what is their importance?

Roberta Teys is the younger daughter of William and Tessa Teys. Gillian Teys is Roberta's older sister. Roberta and Gillian are eight years apart in age and until Gillian ran away from home, Roberta idolized her sister. Roberta was committed to Barnstingham Mental Asylum after confessing to her father's murder. Roberta is significantly overweight and is considered by some to be developmentally delayed. Roberta began eating compulsively shortly after Gillian left home. It was also around this time that William Teys began to molest Roberta. Roberta is an avid reader and kept to herself. Roberta was especially fond of the family dog, Whiskers. However, Simon Allcourt-St. James uncovers forensic evidence proving that Roberta drugged Whiskers before cutting the beloved pet's throat. Roberta Teys does not speak until the final chapter of A Great Deliverance. By the close of the novel, it is well-established that Roberta is severely emotionally damaged by incest and the rage that accompanies being victimized for years.