Rosalind is Dorian's cousin - his equal for good looks and witty insouciance, she's also a lot cleverer than him. She's an ice queen and social goddess, used to getting what she wants.
At present, she rather wants Theo Mackenzie - the slightly rough kid from Lewisham who lives down her corridor. But how long will that last? As Rosalind says to Theo: "Some people have got it and some people have to earn it. I've got it, you've got to earn it."