Cathy's a bit of an oddball here because it's hard to tell what she's like without the bias of the characters coming through. Cathy very rarely gets to speak her mind and we don't really know what she's doing when she's not with our main two couples. That said, there are two big things you need to know about Cathy.
First up, Cathy's basically a symbol of womanhood and bodies for the first half of the book—she's "big and blond and far more womanly than most girls could be comfortable with at age fifteen" (1.22). This is hugely significant when it comes to her accusation of rape against Goodman.
Cathy's totally sexualized from the beginning of the book, and when she says Goodman's raped her, none of the other girls in the group believe her. It stinks of asking for it discourses, as though by being so classically feminine, Cathy can't have been raped—she can only have wanted/asked for what comes her way. She's the woman whose body and experiences keep her from the kind of solidarity that happens in Jules's relationship with Ash. Cathy is a victim of being seen as a threat when it comes to the other girls in the book.
But here's the thing: Cathy's a major example of getting it together. Where Jules sits around whining about how much better other people's lives are, Cathy takes her horrible past and her crushed dreams of dancing and carves out a life for herself in finance—a traditionally male-dominated world.
After the events of 9/11, Cathy owns up to her mistakes and does what's necessary to right the wrongs. She's "brave in her own way" and "unafraid to make demands" (12.69), stronger than most of the group, and of course, manages to be so without their support. The only person she still keeps in touch with is Ethan, probably because he's the only one on her side during the rape fiasco.
So while Cathy kind of hangs on the sidelines in this book, she's a pretty important presence. She clues us into what it takes to hack it in the Jules-and-Ash club (not being romantic/feminine competition), and in a book that centers around someone who basically refuses to evolve (ahem, Jules), Cathy shows just how much a person can take charge of their lives if they want to, no matter what barriers stand in their way.