Satan's true Demon form has not yet been shown in the series, but when he possessed Shiro, he had typical Demon features such as pointy ears, long nails and sharp canine teeth. The possessed version of Shiro also shows features similar to Rin's own Demonic form, these being the black tail with a blue flame at its end and blue flames covering the body.
When first introduced, Satan's mannerisms and laughter bordered on insanity and he enjoyed it when Rin screamed for help while being sucked into the Gehenna Gate. This reveals that his cruel and reckless personality doesn't stop even before his own family. It is further emphasized when Satan said that he created Rin out of mere boredom and after that realized that Rin is exactly what he needed to conquer Assiah as Rin is a human born in Assiah but as Satan's offspring bears Satan's blood.
In the anime he is shown to have been in love with Yuri and seeking to realize her dream of humans and Demons coexisting he attempted to merge Assiah and Gehenna. Ironically enough, he shows no remorse for possessing Yukio and trying to kill Rin even though Yuri loved both of her sons more than anything.
True Cross Academy arc
former that he shouldn't pretend to be his father, Satan possesses the priest. Grinning, he muses on how he has waited for the day when he would finally see his son, before breaking into hysterical laughter and denying the entire thing. He then tells Rin to be happy, as he has come all this way to see him in person. When the boy realizes that he is a Demon, Satan introduces himself, but remarks that he doesn't have time to continue their "touching reunion", as Fujimoto's body won't last much longer under the current circumstances. He then rips off several of the priest's fingers to illustrate his point, using the blood from the severed digits to summon a Gehenna Gate. He tells Rin that he is the only Demon that is able to create this apparatus, and that he will use it in order to drag Rin back to Gehenna with him.
Taking note of Kurikara, Satan picks up the sword, informing Rin that he will need to destroy it first before they can make their trip. When the teenager reacts violently to Satan's approaches by summoning his flames, the Demon king merely laughs off the attempt, grabbing him by the collar and dragging him over to the Gate. As he does so, he explains how even with his supreme power, he is unable to remain indefinitely in Assiah, as everything he possesses ultimately is destroyed. Stating that it was nothing but a whim that prompted him to create Rin, he goes on to say how much of a masterstroke that action actually was, as he now has exactly what he needs to make Assiah his. He then tosses Rin into the Gate, laughing uproariously as he declares that the boy will be reborn, welcoming him to Gehenna. Before Satan's plan could be completely successful, however, he is stopped by the remaining consciousness of Fujimoto, and curses the priest as he temporarily seizes control of his body, grasping his Exorcist pin and thrusting it into himself. As Fujimoto claims Rin as his son, and that he is going to take him back, Satan gleefully exclaims how wonderful it is that Fujimoto plans to commit suicide, as no matter what he does, nothing can escape the Gehenna Gate. After this, Satan is sent back to Gehenna as Fujimoto dies, leaving Rin to escape the Gate by himself.
Powers & Abilities
As the king of Gehenna, Satan is the strongest Demon in existence. His powers are not yet revealed except that he can summon the Gehenna Gate. Like any other Demon, Satan has the power to possess a human body but only for a moment as everything he touches in the world of Assiah is doomed to destruction.
As the king of Demon and Rin's father Satan is extremely strong. He used his strength to snap away Bon's stick when he threw it at him. He used it too when he lift Shiemi's body with one hand and ease above his head and when he punched his own son with powerful blows who caused him very several damages to the face.
Satan has some kind of "sighting" ability. He used it when he showed Yukio was happened to his mother to the past of the history of their meeting. As a Demon we suppose that he could probably do more than this to fool the person he posses.
Satan has the special ability to generate powerful wind from his own voice. We have seen it when he told Rin to "Shut up !!" a great wave of hurricane emerged from his deep voice. He did it too when he knocked out all Rin's friends when they was trying to fight him.
Satan is a very faster opponent but the limits of this ability have not been revealed yet but we have seen that when he attacked Rin he first laughed at him and then did a high speed rush to get him, proof that he has this kind of ability.
Satan has the ability to summon the Gehenna Gate; in the anime, it is implied that he's the only being in Gehenna powerful enough to summon and open the complete and full form of the Gehenna Gate, although other Demons are able to by conjuring a lesser or an incomplete version of the gate. This Demonic door needs blood in order to link Gehenna and Assiah. It's size, appearance, and even abilities depend on the amount of blood offered as a sacrifice.
Being his blood, Rin was sacrificed by the True Cross Order to use his blood to create a large Gehenna Gate which requires a large amount of Demon blood. Combining Yukio's blood with Rin's, the power dramatically increases, creating multiple Gehenna Gates to create a large portal to Assiah.
After his defeat from Rin and Yukio, Satan was rejected from his vessel, but possessed and took control of Gehenna Gate, giving more abilities to send gate Demons, shoot beams from its eyes, and rise into the sky so it can attack the city. This Gate is regenerative and able to withstand Rin's flames. Its weakness however is its heart which Yukio and Rin pierce through to destroy Gehenna Gate completely.
Like Rin Satan is highly durable, even maybe more than Rin because of being his father. He could stand a fight against his son's friend without getting tired. He could face Shura's powers too and still standing without difficulty. Rin can't uses his flame during a long time with ease he has to practice that, however, Satan doesn't need too.
Like Rin, Satan can control and manipulate blue flames at will. He is able to ignite his victims in blue flame by spontaneous combustion (activated by eye contact). He also has the ability to shoot multiple fireballs from his palm during his battle with Shura and can burn out any barrier that was created by the Exwires. When fighting Rin, he ignites him with his flames but Rin, however, appears immune to it. This was only shown in the anime.
- In the Abrahamic Religions, Satan is the name given to a fallen angel who fell out of God's favor after waging a war in Heaven. He is also the personification of all things evil, as he leads humanity astray with deception and temptation.
- (To Rin Okumura) "I'm a perfect entity with ultimate power, but there's one thing I lack. Do you know what it is? It's a substance in Assiah strong enough to contain me! Everything in this world that I touched is doomed for destruction!"
- (To Shiro) "Curses what do you think you're doing? ...So you mean to sacrifice your life for him? You astound me, you wretched priest. However, it's already too late. Gehenna Gate has taken a hold of you, and it will never let you go."
- (To Rin Okumura) "Oh, it was out of mere boredom that I created you... but I have since realised, it was a masterstroke! You exist in the world of Assiah... and yet you bear the blood... of the god of Gehenna...! You are exactly what I need to make Assiah mine!!"