Ley had been born weak with an incurable disease.
Her father, who was desperate to save her, called upon the great wizard Oz. Through the transplant of some ancient female demon's core Edna, Ley was miraculously able to survive.
When the core was transplanted into Ley, a ritual to summon the soul of the core's owner was performed. Because of this ritual, Ley now has another soul inside her.
she was once overtaken by Edna because of Duel, but Dio has brought Ley back to her body