Emily is the Alpha Vampire's "special girl".
Emily was abducted by vampires in about the year 2000 and kept by the Alpha Vampire as a "special girl", meaning she was one of his virgins that he fed on as a delicacy. During this time, she was brainwashed into thinking of the Alpha as her father.
In 2012, Sam, Dean, and Bobby's ghost find Emily at the vampire's lair in a secret room behind a wall. Sam says he'll take her back to her mom, whom she apparently doesn't remember. Emily tells them the dead vampires in the next room once brought back three humans who gave up rather easily. But, when the vampires fed on them, they died. One vampire didn't feed and went hunting for animals.
Emily says that she knows of a retreat that the Alpha Vampire keeps and takes the Winchesters there. They decide to keep her in a motel room to keep her safe, but as soon as they leave to get the Alpha Vamp's blood, she calls her "father" and alerts him that the brothers are on their way. Later, she tries to defend the Alpha from Sam and Dean, and he comes to her defense. In the end, he still gives them his blood to use for the leviathan weapon.
- Emily's actress Laci J. Mailey later went on to portray Deputy Jenna Nickerson in Out of the Darkness, Into the Fire and Form and Void in season 11.
- It's not known what the Alpha's true intentions were for her, if he really did plan to simply drink her blood when she was old enough, or if he actually intended to turn her into a vampire. In any case, the Alpha does seem to care about her well-being.