Who is Ellie McCrae from About a Boy and what is their importance?

Ellie McCrae is one of Marcus' classmates. She is described as a troublemaker who often finds herself in the head mistress's office at school for minor offenses. Ellie is a Kurt Cobain fan. Marcus finds Ellie exciting and convinces himself he is in love with her until the day Kurt Cobain dies and Ellie breaks a music store window.