A fisher is a person, often a female, who constantly seeks out, or "fishes," for compliments. They often wish for reassurance and therefore say the opposite of what is typically blatantly obvious in order to make the person they are fishing from feel badly, and then console the person and compliment them. The fisher will say, "Really?" as if they don't already know, and then feel better about themselves. It's a silly ego-booster tactic, but can be found absolutely anywhere, and you probably have done it at some point.
Girl 1: "Karen keeps telling me how fat she is. Is she aware that she's emaciated?"
Girl 2: "She's just fishing. Next time, tell her she looks like she's packed on a few pounds."
Boy: "I have no friends."
Girl: "Aww, yes you do! You have me and Chloe and Manda and Ryan and..."
Boy: "Really? They like me?"
Girl: "Stop being such a fucking fisher."