Satan is the first and most powerful demon. He was sealed inside Jio Freed for the causes of the Kabbalah. He's the reason that half of Jio's hair and eyes are a different color(His hair being white opposed to its regular black color;his eye is red rather than blue).Whenever Jio is heavily injured and/or close to death, Satan offers some of his power.
First form: Jio's hair turns all white, his eyes both turn red, and his power is doubled. Even in this form, it's difficult for Jio to control it. Satan takes over Jio's body, with Satan speaking instead of Jio. There are some drawbacks, however, as Jio loses consciousness and can be vulnerable to attacks.
Second form: Jio keeps the all-white hair and red eyes, and his strength is quadrupled.