Little is known about Emily's life but it's safe to assume she was an extremely vain woman.
In death, Emily went to Hell where she came to embody the Sin of Vanity. Her sins in life are unknown were likely considerable in order to cause her to become a Sin upon death.
Upon entering Hell she was cursed into becoming unbelievably hideous, gaining the appearance of a being that's been literally turned inside-out.
This transformation seemingly drove Emily insane and she now wanders Hell, attacking and skinning anyone she finds and wearing their pelts to "look pretty again." When not wearing the flesh of others, Emily uses a tattered cloak to conceal her deformities.
Emily has not shown the godlike powers of some other Sins yet but has displayed the ability to mimic the appearance of the owner of the skin she is wearing and some level of healing abilities. This lack of power, coupled with her apparent mental instability, has caused the other Sins to regard her as being somewhat pathetic.
Despite this, Emily is seemingly aiding Dr. Kane in his grand plan - though what this plan entails is still largely a mystery.
It has recently been revealed that a fallen angel named Aecas has been abducting mortals from Earth and stealing their energy in an attempt to make Emily beautiful again. Aecas' apparent motive for doing this is that he has seemingly fallen in love with Emily.