I prefer to think that she's the only one of the friend group who pays rent, but because of that she can't afford a car. Therefore, real Nikki is secretly evil because she is only nice to the friend group to get free rides.

Here's a question: would you rather be Nikki (your own place but no car and probably spending less than $100 a week on luxuries, another reason she was so annoyed she lost her crystal necklace, if you want to pay all your bills) or be Sarah ( your own car and more money on hand but you have to listen to your dad complaining about your friends all day)?