Snow is a girl who Joseph saved in the past and goes to bring about Joseph to his senses while sustaining heavy injuries in the battle with him. Eventually, she succumbs to Joseph's power and reverts to human form. Joseph continues his attack until Snow speaks with her last words: "It's not your fault; none of this is your fault. Don't be afraid, just live." Joseph said the same thing to her. He then reverts to his human form as Snow slowly passes away. She seemed to show some affection for Joseph as seen in episode 19. (Source: Wikipedia, edited)