If the targeted creature is a double-faced or melded permanent, Identity Thief copies the face shown. Because Identity Thief is not a double-faced card or meld card, it won't be able to transform or meld. If it copies a melded permanent or the back face of a double-faced card, its mana value will be 0. Exiled double-faced cards and exiled meld cards will return with their front faces up.