secret hater

A really close friend who secretly hates on you or wishes for you to not succeed in anything. This person usually will not show that they are hating on you to anyone and will be perceived as a really good friend to you. They also might try to keep you down or assist in your failures secretively. Or have malicious thoughts of what they could do to make your life miserable.

Mary and Jane, best friends

Mary: (pulls up in her new car outside Janes' house) "I finally got my new car!! Wanna go for a ride!!??"

Jane: "Hell yeah!!!" she shouts in delight. But Jane is really thinking she wishes Mary would get into an accident and ruin her new car. Jane is a secret hater.