If I was in this season, I'd probably spend most of my time trying to teach Kat how to speak proper English in the hopes of recreating a Henry Higgins/Eliza Doolittle relationship that would let me get away with being a dick to most of the house.
Things have come to him in his life, no need for him to put on or win people over, people try to win him over for the most part, but it doesn't work in the BB house because everyoyne is on an equal playing field. Grace was a social chameleon and adapted well to her environment whereas Rex does not.I agree but I think a lot of it is that Grace was a "hot" girl, who usually end up the centre of their clique (e.g. Saskia, Jen), particularly with guys like Glyn and Luke desperate for female attention that they'd never get in their regular lives.
The only good thing about Grace leaving early is that she tricked the country into loving her in that rubbish circus reality show <3
or Orlaith 
