One who acts as if they are your best friend, but when asked non-surface level questions, refuse to answer.
An anti-friend will also ask you to divulge all your deepest secrets, then go behind your back and tell everyone.
Friend John: Dude, who did you fuck this weekend?
Friend Steven: Fuck you man, i don't kiss and tell.
Friend John: But, I thought we were best friends... you're being an anti-friend.
(n.) - A person who portrays themselves like a true friend or confidante but, in reality, don't approve of the person or their actions. Although, they act generally friendly towards said person.
Yeah. Even though Jason hangs out all the time with him, he's always talking about how Trevaren lies and is extremely condescending towards everyone. That's an Anti-Friend.