Ginny was never the same after being possessed by voldemort in the chamber of secrets which of course harry saved her. Her mind was twisted and she grew an unhealthy obsession with harry but kept it under control. But after she saw harry making out with Cho Chang and hearing about Harry's past with the Dursleys. Something in her snapped and she decided that harry was her's and hers alone. He didn't have a choice and if anyone tries taking him from her, they would be delt with (I DO NOT OWN HARRY POTTER J.K ROWLING DOES)
4 parts