The characters involves a psychiatrist/doctor, a girl and her sister. I guess the story goes like this - The sister commits suicide by jumping off a building. The girl tries to find out what caused her sister to do something like that. She ends up meeting a psychiatrist who used to treat her sister.
The girl gets hypnotized by the doctor and he informs her that she has got a twin personality (and I guess he tells her what the other personality's name is). She did not believe that first. But next morning she finds the name written near her bed.
Towards the end, girl finds out that the doctor manipulates girls' minds during hypnotism. His soul then enters the girls' bodies and uses them for his pleasure. I remember something like - causing pain to the body will break the connection.
Finally it is revealed that it is not the doctor but his physically challenged brother who is doing all this. The doctor's body is occupied by the brother's soul most of the time. During his transition into a girl's body, the doctor is locked up somewhere. I think the girl found this out through a letter from the doctor.