She was played by Nina Siemaszko
Dr. Amelia Emerson is a guidance counselor at the high school where titular protagonist Molly Hartley attends. In actuality, Dr. Emerson is a servant of the Devil (or possibly the Devil herself), and it was revealed that Molly's parents made a deal with her to save a newborn Molly after she died during birth. The deal had Jane and Robert Hartley raising Molly as their own until her 18th birthday, when she would forever belong to the Devil.
In the climax of the film, the evil Dr. Emerson appeared at the home of Molly's boyfriend, Joseph, and revealed her true demonic identity to Molly. She later presented Robert, captured by one of her minions, to Molly, and even attempted to manipulate Molly to kill her father, stating that it could break the satanic pact. Instead, Molly plunged the knife inside her own body, but seeing as she turned 18 at midnight, she couldn't die. The film ends with Dr. Emerson congratulating Molly (now accepting her fate) and Joseph at their graduation.