Here are the various forms of Satan that Jio transforms into.
This form appears in chapter 1, his hair turns a glowing white, both of his eyes turn red, giving him a more demonic look. A mark appears on his forehead that resembles ancient markings, which means 666. Both his power and speed grow miracously.
This appears exactly like his initial transformation but has the actual 666 sign on his forehead. But the more power his beast gives him he can also sprout six wings on his back. His arm starts to transform into the beast itself.
The appearance this gains is same as all the former transformations, but with some details still to entail. His skin becomes crinkled, like cracked kind of. His mouth becomes slitted so it becomes bigger, giving him more of a demonic look. He also, if has enough of Satans power, can even sprout six skinny, long tail-like thing from his lower back. His arm begins to turn more and more like the beast.
Although he never acquires this form completely, he still gains half of its power, if not more. His arm begins to turn more and more like the beast, with this, if he continues his demon forms progressively, his arm will turn into the beast itself, shown as when Yuria did not transform into the beast herself, only on her arm.