Born a mere two days after her conception, Emma aged fast, as was customary for Amazons until she reached 16 years of age whereby the aging process became normal. While still in her crib as a two-year old, she was able to communicate with her mother by talking.
When she grew up to 6 years old in a few hours, her mother sent her away with the Amazon leaders who had her undergo intensive training, including consuming human flesh with milk and being branded with the mark of the Amazons before being sent to kill her father to complete her initiation.
Landing on Dean's front door, she tells him that she escaped the Amazons as she didn't want to be what they had planned for her. She tries to enlist Dean's help to stay off their radar. Dean seems to sympathize with her but when she tries to sneak up on him with a dagger in her hand, he turns around with his gun at the ready. She tells Dean that he won't be able to do it but Dean says he can.
Meanwhile, Sam has returned from the professor's office after learning that she was going to kill Dean and is looking through a crack in the door. Seeing that Dean is hesitating, he bursts in and Emma turns to face him, flashing him her red eyes before turning to Dean and pleading for his help. However, as she turns again to face Sam, he shoots her in the chest, killing her.