Although little is known about Emily's life, one can assume she was an extremely vain woman. In death, Emily went to Hell where she came to embody the Sin of Pride/Vanity. Her sins in life are unknown. Her punishment in Hell was to become unbelievably hideous, gaining the appearance of a being that's been literally turned inside-out.
This monstrous transformation seemingly caused Emily to become neurotic. 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 (not unlike Jack's) to conceal her deformities.
Despite being a Sin, Emily has shown no real powers except 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, the Sin of Envy, in an unknown plan.
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.
Due to her extremely decayed appearance, it is practically impossible to tell what species Emily is.