Bloody Smiles (Jeff the Killer x Reader)
When (F/N) was younger, she suffered from multiple-personality disorder. This was odd for a number of reasons; one being that usually this disorder is rare. In spite of that, MPD was usually diagnosed in adulthood. One of the personalities was just her usual self; the other one was insane and psychotic. Eventually, the medication and therapy sessions had got rid of her disorder. Unfortunately, it was the sweet, sane side of (F/N) that had been ridden, so she is now trapped in a crazy state of mind. This made her the killer she is today.
(I am well aware that this plot is not accurate for anyone who wants to point that out.)