A Koi Fish is a man who gets overly attached to a girl and refuse to leave her alone when she has no interest in him. They generally become psychotic and ruin their friendshipsbecause of this. Some of them try to ruin their targets lives.
In Japan during world war II a group of homosexuals got the Koi Fish tattoo on their left arm. Now commonly used in Japan and America as a more verbose way of letting people know their sexual orientation.
Person 1: "My friend just got a Koi Fish Tattoo on his arm."
Person 2: "So how long have you known?"
Person 1: "He has been out of the closet for a couple years now."
Person 2: "Good for him."