Ivy used to be a Plain Jane along with Melinda, Nicole, and Rachel. Ivy wasn't at the party where Melinda called cops and doesn't seem to have any reason to be mad at Melinda. But she's not exactly bubbling over with friendly overtures either. As with Nicole, it's not clear whether Ivy is avoiding Melinda because she's unpopular, or whether they have just drifted apart. Regardless, Ivy proves to be a good friend to Melinda over the course of the semester.
Art seems to be the thing that draws them together. Ivy and Melinda's connection is cemented when they collaborate on the Andy Evans problem. Melinda never flat out confides in Ivy, but she hints strongly that something happened. Ivy and Melinda's friendships seem clear and unambiguous. They genuinely appreciate each other and have similar interests and outlooks. This friendship stands in stark contrast to the friendship between Melinda and Heather.