In the manual of DMC4, it said that Nero had gotten attacked and the scratch developed into his Devil Bringer. What if Nero is actually MORE demon than human instead of the opposite? A dominant gene of his would be that if he is sufficiently injured by a demon of great power, the injured area would absorb the power to compensate for the injury, thus turning that part of him into a fully demonic part. It could allow Nero to turn more and more demonic while the game progresses. Also, a recessive gene of his would be the specter DT, meaning he'd still keep it even as a full demon, thus allowing him to have a super costume.