Satan

Satan is the king of Demons who rules over the world of Gehenna. He is also the father of Rin, Yukio, and the Eight Demon Kings.

Appearance
Satan's true form within Gehenna appears as a living flame with multiple eyes atop a pillar of intertwining, demonic bodies; but while in Assiah, he takes on the physical traits of his host with some minor alterations. While possessing Shiro Fujimoto, his body gained long, pointed ears, sharp teeth, a long tongue and claw-like nails with several balls of Blue Flames surrounding him. When possessing Goro, a vessel better suited to him, his transformation was more subdued consisting of slightly pointed ears and teeth with two horn-like flames hovering around his forehead and a long, light coloured tail with a flame at the end.

Hosts
The various hosts of Satan have of course possessed their own appearances. The list of those he possessed and the circumstances surrounding them are as follows-:

Personality
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 manga, upon taking over Goro's body and developing his own consciousness, Satan, at the time known as Rinka was described by Yuri as being like a very powerful baby. He lacked any any real moral compass and had no concept of right or wrong, thus he did not comprehend the moral ramifications of killing and wanton destruction. Despite this, he clearly cared for Yuri. She was the only one he would listen to and he was shown to be immediately jealous upon meeting Shiro. Satan was also shown to be extremely competitive, attempting to kill Shirou after losing a game to him vowing to beat him. He also had an incalculable intellect as he learned at an accelerated rate.

After gaining a more awareness and developing an ego, Satan gained a god complex, after realizing he was smarter and stronger than any human and demon alive to the point where he'd kill anyone around him or Yuri. Despite this, he maintains his obsession for Yuri, to the point where both of them made love, which resulted in her conceiving a child, however without his knowledge of the latter. At the "betrayal" of Yuri, which was Lucifer attacking him while Satan was in a weakened and erratic state due to his Goro's body breaking down, resulting in his hatred toward his former love and his current mindset.

Anime Exclusive
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.

History
Record of Satan's existence is actually quite short, with his ego only sprouting a few years before the Blue Night. Before, he appeared as nothing more than small specs of blue light, and was called a "Rinka", along with many other names depending on region. He would randomly appear around the world with no reasoning, thus causing many to have different interpretations of him. Many different descriptions of a Rinka included dead souls, the fires of hell, fairies, and temporary forms of sacred foxes. During this time, Rinka had no conscious.

Rinka was first fully brought into Assiah roughly 19 years ago. His ego sprouted into the strongest clone the research facility made, that clone was meant for Lucifer but unbeknownst to the other Demon Kings, they had no idea who it was that possessed the body meant for Lucifer. By the time they realised, it was too late. Rinka escaped the facility and quickly found Yuri. After being taken back to the facility, he was looked after by Yuri, as she was the only one he would listen to. Yuri would being him books and toys, but after 1 year and 3 months he started becoming bored with them. In an attempt to expand how world view, Yuri brought Shiro along with her to play with Rinka. The demon was immediately distrustful of Shiro and grew to hate the exorcist quickly.

The next day, Rinka took a guard hostage and demanded to be given more books for the guard's safety. After several hours Yuri approached Rinka in an attempt to convince him to let the guard go, only for the demon to nonchalantly kill the guard and explain to Yuri that he had come to the conclusion that he was the Demon God Satan. Fleeing from Section 13, Satan was in hiding for at least sixth months before his body began failing, prompting his return to the Asylum.

After a month long siege, Yuri returned to Satan's side, during which time the Okumura twins were conceived. Attempting to flee from Asylum, Satan's body finally gave out, leaving him crippled until the Blue Night; About nine months later, when Yuri Egin was giving birth to her children and the mass slaughtering of exorcists around the world took place. 

True Cross Academy arc
Taking advantage of Shiro Fujimoto's temporary weakness after his argument with Rin Okumura, in which the latter told the 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.

Blue Night Investigation Arc
Satan is first seen as a small blue flame called a Rinka who often played with Yuri as very young child. Mephisto notes that Satan probably couldn't tell that he was playing with her since his ego had not sprouted, but does not the fact that she is most likely his first intelligent interaction. During the instance he is seen playing Yuri, she is suddenly called to return home by her guardians. Not too long after this, the shelter in which she said is found caught on fire, with the bodies of guardians found burned to death. It can be implied that Satan had burnt the area. He is quite absent after this, but still seems to follow Yuri around, as he would appear randomly throughout her years at True Cross.

Satan makes another major appearance when Yuri is 16 years old. The girl and Shiro have an argument in Mexico after Shiro elbows Rick in the face and proceeds to degrade him. This angers Yuri, who eventually slaps the exorcist, calling him weak for taking his problems out on others. Shiro tries to retaliate, but is stopped as a blue flame suddenly appears and guards Yuri. Everyone around is shocked at the incident, but eventually lets it go.

Later, Yuri is seen at a demon farm, and is confronted by Shiro about the previous incident. After the latter shares his experiences in asylum, the former sympathizes with him, and kisses him and follows it with a confession. Shiro rejects Yuri, leaving her dispirited. Satan appears again immediately after the situation, almost as if to comfort her. Not too long after this, Goro's eyes are seen to open in the tank, showing Satan's first attempt at possession.

Mephisto explains that because Satan had not yet developed an ego, possession was quite complicated. As a result, it takes about 9 years for him to adapt to possessing Goro's body. During this time, the numerous attempts of possession and constant exposure to elixir strengthened the host bodies strength. This made the body highly suitable for Satan. When he makes his first successful possession, those under section 13 are left confused, as they had never seen this kind of demon at this kind of strength before. Mephisto decides to keep the situation a secret and hold Satan under confinement. Shiro Fujimoto is eventually ordered to observe him. When he arrives, Shiro is shocked to find that the body possessed is his brother. Satan is immediately angered at the sight of Shiro. Consequently, he flails and screams, and even flares up in flames and kills those standing near him. He then escapes the facility.

That night, Yuri is heading home after a mission. The sound of footsteps catch her attention, and eventually finds Satan in his new state heading in her direction. At first, the woman is anxious because she initially thinks that it is Shiro. Because Satan has had no exposure to the whole ordeal, he makes several attempts to speak but fails. He eventually utters out Yuri's name, causing her to calm down. She then acknowledges him as Rinka. Section 13 arrives at the scene, and takes hold of Satan. They order Yuri to follow. They hold Satan under confinement once again, and question Yuri.

Once more, Satan attempts another attack and escape, but is quickly stopped by Yuri at a simple command. Because of this, the exorcist is put to work for section 13 and observe Satan, as her experience with him is proved to be very helpful. She spends months educating him about mathematics, reading, history, and even feeds him. She acts almost like a mother. Throughout this time, he is shown to be very attached to her, and acts very much like a child, throwing tantrums and crying to her aid. His language is child-like as well.

Concerns begin to grow. As Satan learns things exceptionally fast, his sense of emotion and reasoning is lacking.

Powers & Abilities
As the God 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, with even the strongest bodies only lasting five minutes at most

Superhuman strength
As the God of all Demons, Satan possess immense strength, giving him the ability to carelessly drag someone like Rin with ease and rip off his own fingers with no difficulty.

Gehenna Gate
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.

Anime-Exclusive Content
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.

Superhuman Durability
Like Rin, Satan is highly durable; maybe even more than Rin, due to his being Rin's father and a demon. It was shown that he could stand a fight against his son's friends without feeling exhausted. The same thing happened when he faced Shura's power as well. However, unlike Rin who cannot use his flame in a long period of time with ease and has to practice for that, Satan does not seem to have any difficulty in this.

Fire Manipulation
Just as his son, 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.

Infernal Wind
Satan has the special ability to generate powerful wind from his own voice. It was shown in the anime when he told Rin to 'shut up'; a great wave of hurricane suddenly emerged from his deep voice. He did the same when he knocked out all Rin's friends as they were trying to fight him.

Superhuman Speed
Satan is a very fast opponent but the limits of this ability has not been revealed yet. However, it has been seen that when he was trying to attack Rin, he initially laughed at him before using a high speed rush to get to him; a proof that he may have this kind of ability.

Trivia

 * 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.

Quotes

 * (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? Damn you exorcist!! ...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!!"