Who is Shay from Uglies and what is their importance?

Shay is a fifteen year old girl at the beginning of the novel who is about to turn sixteen and face her own operation. Shay expresses some doubts about the operation to her friend, Tally. A week before she is to turn sixteen, Shay decides to run away to a place known as the Smoke. Shay leaves instructions behind for Tally to follow if she changes her mind about joining Shay in the Smoke.