Rachel and Melinda have been best friends since grade school at least. She's important to the story because she's important to Melinda. Before we know Melinda was raped, Melinda thinks, "If there is anyone in this entire galaxy I am dying to tell what happened, It's Rachel" (1.10). Unfortunately, Rachel has totally turned against Melinda. She, like everybody else, seems to believe that Melinda called the cops at the party just to get everybody in trouble.
Like Melinda, Rachel changes a lot throughout the novel. Many of these changes are beyond our reach, because they are beyond Melinda's. Although Rachel is an important character, she doesn't get many lines, and everything we hear about her is from Melinda's point of view. This leaves readers with lots of room to speculate about her motivations and to explore different reasons that might drive Rachel's actions.
Rachel's willingness to believe the worst of her friend isn't endearing. We know she sees Melinda right after the rape. It would have been obvious that Melinda had been through some kind of attack, right? When Melinda tries unsuccessfully to reconnect with Rachel in the school bathroom, Melinda thinks,
I don't want to be cool. I want to grab her by the neck and shake her and scream at her to stop treating me like dirt. She didn't even bother to find out the truth – what kind of friend is that? (9.10)
Good question. Rachel's quick and easy betrayal of Melinda makes Rachel seem weak and disloyal. When she starts dating Andy Evans, she seems like a bad judge of character to boot.
But there's another way to look at Rachel. Like Melinda, she's just starting high school and she's not sure where she fits in. She's trying to find her identity. She's experimenting with different cultures through her friendships with foreign exchange students. She's just as flattered by attention from a handsome senior as Melinda was when she first met Andy.
In all fairness, if Melinda had told Rachel from the beginning, Rachel probably would have stood by her, or at least believed her. For Rachel, the fact that Melinda doesn't explain why she called the cops on a party that Rachel brought her to might seem like something of a betrayal.
This doesn't excuse Rachel's action. Her silence toward Melinda might show that she doesn't take their friendship seriously, or that she, like Melinda, can't express what she feels in words. We might also keep in mind that like pre-Andy Melinda, she's probably innocent about rape. Maybe it's beyond the scope of her imagination that Melinda was raped at the party. Or maybe she had the suspicion all along, but pushed it away.
In many ways, the truth about Andy, and about Melinda, frees Rachel just as it does Melinda. When Andy can't keep his hands to himself at prom, Rachel almost slaps him. This is after Melinda finally confides in Rachel about the rape. Although Rachel doesn't seem to believe Melinda's accusation, it still warns her and puts her on guard around Andy. This suggests that that Rachel has some clear boundaries about her body and isn't afraid to fight for them.
At the end of the novel, we learn that Rachel tries to reconnect with Melinda after Andy attacks Melinda a second time. Now that Rachel knows the truth, her world can make sense again too. She can understand Melinda's actions, and she can look back on her time with Andy and see all the signs that showed he's not a nice guy. Hopefully, she also learns something about loyalty and friendship.