Who is Gracie from Delirium and what is their importance?

Gracie is Lena's cousin's youngest daughter. She has lived with Aunt Carol since her father disappeared, and her mother died from a heart attack. Gracie never speaks, and most people believe that she is mentally handicapped. Lena, however, knows that Gracie can speak and grieves for the loss of her mother. Gracie often crawls into bed with Lena at night. She is also aware that Lena is sneaking out at night. In the end of the novel, Gracie unties Lena to allow her to escape.