I'll also get to the boys at Easton, too. Josh Hollis, Thomas Pearson, Dash McCafferty, and Gage Coolidge are the boys constantly in the series (except for Thomas, but Thomas plays an important role in the series).
I'll start with Ivy Slade. Ivy Slade returns to Easton as a senior in the second arc of the series, when Reed is a junior. She had left Easton the year before. She opposes everything Billings stands for, and hates the Billings girls with a passion. She was invited to join Billings at the end of her sophomore year, but she declined. She is an equal to the Billings girls in beauty and status, though. Ivy becomes Reed's enemy #1, believing that Ivy is determined to ruin her and Billings in the process. She has dark hair and dark eyes.
I think of Ivy as Elizabeth Gillies as Jade West in Victorious. Like Jade, Ivy is sassy. Even though Elizabeth has blue eyes, I can see Ivy look like her. Both wear a lot of black, both look like rebellious girls who don't fit the norm, etc.