Emma Naylor is the hidden main villainess of "The Rich Girl", book #44 of R.L. Stine's original Fear Street series (published on April 1, 1997).
Emma was the best friend of Sydney Shue, and the two girls worked together at Shadyside's local movie theater. Emma's parents divorced when she was young and she was left in the care of her mother, who was a struggling waitress and lived in the Old Village (the poorer part of Shadyside). She also had an injured knee which she had no money to get surgery for, and Emma was trying to work more shifts to earn enough money to pay for it. Emma was a complete contrast to her friend, whose parents were wealthy and lived in the lavish North Hills.
When Emma and Sydney found a bag filled with money behind the movie theater while taking out the trash, Emma goaded Sydney into holding onto the money until they could see if anyone was looking for it before splitting it among themselves. When Sydney told her boyfriend Jason Phillips about the money, it soon after appeared that he was trying to kill Emma to get a hold of the money, first by pushing her down the stairs at school and again by cutting the brakes on her car. To appease him, the girls offer him a share of the money and take him to the Fear Street forest to show where they hid it. But there, a struggle seemed to ensue between Emma and Jason, during which Emma killed Jason in self-defense. After some pressure from Emma, Sydney helped her hide her boyfriend's body in the forest. But afterwards, it appeared that Jason had rose from the dead and began tormenting Sydney, eventually breaking into her house and confronting her as she broke down.
Emma's villainous reveal came in the final chapter, when it was revealed that Sydney had been committed to a mental hospital after her breakdown. Emma was shown waiting in the lobby until Sydney's doctor came out--along with the very much alive Jason. After putting on a show of grief for the doctor, Emma and Jason left the hospital and gleefully talked about Sydney's condition, revealing that the two had colluded to drive Sydney insane so that they would be free to have the money all to themselves without the risk of Sydney trying to turn it into the police, faking all of Jason's supposed attacks on Emma and his death in order to accomplish the deception. After leaving the hospital, Emma and Jason retrieved the money from the woods and went to the mall, where Emma tried to use the money to buy an expensive leather jacket. But when she did, the cashier laughed and gave Emma a shocking revelation--the bag only contained gag money.