Question: What exactly is wrong with Joon? I don't think they mentioned it in the movie, but does anyone have a good guess?
Answer: Initially, I thought that she had PTSD after her parents died because she appeared to have watched the accident or been in the accident.
Answer: She had schizophrenia, she showed that with her paranoia when she accused Benny of cheating and hallucinations when she was getting upset with the voices in her head on the bus.
Answer: I would guess high functioning autistic. Similar to Rainman. Especially her being obsessive compulsive.
William Bergquist