Who is Maggie from The Carrie Diaries and what is their importance?

Maggie is Walt's girlfriend when the novel opens. They eventually break up, and Maggie starts dating Peter.... a nerd, who due to Maggie's influence becomes the coolest guy at school. Maggie smokes like a chimney, drives her mother's big Cadillac around town, and is only happy when she is the center of attention.