Here are some possibilities:
* Moaning Myrtle: She was a student who died in a bathroom, and became the resident ghost. Her death is not explicitly described, but it's implied to have been an accident.
* The Grey Lady: Her name is Helena Ravenclaw. She died of a broken heart after her daughter stole a diadem.
* The Bloody Baron: He was a wizard who killed his girlfriend and then took his own life, becoming a ghost.
Could you provide more context about this "ghost girl" so I can help you further?