I don't think he's possessive of her. He is trying to keep his group intact for the sake of survival. He hasn't stopped her from talking to other people. Just with Vishal because he felt Karishma could be influenced by a guy. I think he mentioned to Mink once about another good looking guys being in the house besides him (Vishal).
Niketan has been on Vishal's case ever since he came. Even before Karishma came into the picture. Niketan clearly has a problem with Vishal.
I was actually surprised when Niketan was upfront with Karishma about her being diplomatic with people in order to survive evictions (he does the same though). And when she brought up Sana he threw the fact that it is Sana who doesn't want to be friends with her, in her face. She looked quite stunned. So props to Niketan for that!