Ellie McCrae from About a Boy is a friend from Marcus's school. He is convinced that he is in love with her. She is a rocker-girl wanna be and wears a lot of rockband t-shirts, especially Kurt Cobain. Marcus decides that he isn't in love with her so much when he sees her violent reaction to Cobain's death. He realizes that Ellie has too much baggage and issues for him. He still considers her a friend, though.
Ellie mcCrae is quite possibly the most important character in the novel for truly allowing Marcus to become rebelious, as demonstrated in chapter 19 whereby ellie and marcus meet and resultantly marcus storms out of the headmasters office.