We're going to take the words right out of Cassie's mouth to characterize Rosalind: "No conscience, no empathy, pathological liar, manipulative, charming, intuitive, attention-seeking, easily bored, narcissistic, turns very nasty when she's thwarted in any way…" (23.183). Cassie hits the nail on the head with this assessment—and we're pretty sure she'd like to hit Rosalind on the head, too. Hitting aside, though, Rosalind may seem like a charming young woman, but she's walking proof that appearances can be—intentionally—deceiving.
Although it's Damien who actually kills Katy, Rosalind's sister, it's Rosalind who manipulates him into doing it, and we're not sure what's worse. Rosalind is either jealous of all the attention Katy receives (Katy's a dancer; Rosalind is, well, nothing), or just bored, but no matter what reason, she convinces Damien to kill Katy—her own sister, let's remind you—and does it so well, Damien thinks it was his idea. That's like, Mensa-caliber manipulation skills.
To make Rosalind even scarier, she's only eighteen. Oh, scratch that—she's seventeen. She lies about her age, too, so that when she finally confesses, it's inadmissible. A seventeen-year-old can't officially confess to anything without her parents present, and by wrapping Rob around her little finger and lying about her age, she manages to manipulate the entire Murder Squad, too. Rosalind is officially the meanest of the mean girls we've ever seen. Regina George should be shaking in her boots.