Jennifer O'Brien was a Djinn, and for nine years she had been working at the coroner's office, first as his assistant and eventually becoming a coroner herself. Using her position, she disposed of her victims' bodies. She was also a rare type of Djinn, which feed on fear rather than blood.
Upon first meeting Dean Winchester and Charlie, Jessica seemed a strict stickler for the rules, and refused to let them see the bodies without the correct paper work. However the real reason was that she had already had the bodies burned. The bodies were the victims of her son, who had messed up, leaving the bodies behind.
Later while Sam, Dean, and Charlie were sneaking into the coroner's office, Jennifer arrived at her office. Charlie distracted her, first by asking for a copy of the forms they needed to have signed, claiming the local office had burnt down. In an attempt to distract her longer, Charlie asked her for fashion advice.
What Charlie was unaware of was that Jennifer could smell her fear, and decided that she had enough fear to be a meal for both her and her son. She broke into Charlie's apartment, kidnapped her, and took her to an abandoned warehouse that she owned.
When Charlie accidentally revealing she knew what Jennifer was, Jennifer guessed that they were hunters, and intimidated Charlie before infecting her with her Djinn poison. Sam and Dean tracked her down, and went searching for her and Charlie. She confronted Sam, in his deteriorated condition, and beat him down, when Dean, who snuck up behind her, and stabbed her through the heart with his silver knife dipped in lamb blood, killing her.