One stage above a stage 5 clinger. This person did not derive their obsession with the opposing person through sex, but simply through a random act of kindness. If you have ever been called at 4am and the other person "just wanted to talk" you have a stage 6 clinger on your hands.
Fred: Jane is such a stage 5 clinger.
Jack: Did you guys have sex?
Jack: Well then she is definitely a stage 6 clinger.