Brooke is a beautiful girl. And, well, that's about it—that's pretty much all we know about her. She fills the role of female love interest for Will. Unlike Will's other friends, who primarily exist to assist him, Brooke gets to both assist and kiss the hero.
From the moment Will sets eyes on Brooke, he starts thinking about her in terms of beauty and score-ability. For instance, when Todd makes Brooke cry, she runs to her room. The first thing Will thinks is this: "She lives here. There is a God"(13.58). Not, "I hope Brooke is okay." Or, "I should check on her." But, "This will make it easier for me to score."
Will gets nervous around Brooke, but he remembers his dad's rule: "Never be nervous when talking to a beautiful girl. Just pretend she's a person, too" (12.86). Well, if you have to "pretend" your romantic interest is a person, we're not sure if we can help you, but maybe Will will learn eventually to get past that point.
Anyway, Brooke's role is to take care of Will—for example, when Will risks Brooke's safety and her scholarship at the school by forcing her to hide his contraband cell phone.
When she isn't taking care of Will, or crying, Brooke is getting kidnapped and held in a boathouse. She is the Princess Peach to Will's Mario—except Princess Peach is better at golf and tennis.
We're being tough on Brooke, and we feel a little bad about it, because she's so darn nice. She helps show Will around the school, and she helps him research the Paladins. Without Brooke, Will might not have found his place at the Center. And with all her friends developing superpowers, Brooke feels insecure that she's normal, like a late-bloomer in a world of supermodels. Emphasis on the super.
It's suggested at the end of the book that the roommates come into their powers at different stages, just as people develop at different ages. Will Brooke get her superpowers later? And how will Will react if that happens?