Satan is the ruler of Gehenna. It has been said that he is the creator of every demon in existence. Satan wants to merge Assiah and Gehenna to make one joint world, but cannot fully exist in Assiah because there is no vessel strong enough to contain him and the antagonist in Blue Exorcist.
Satan's true demon form has not yet been shown, but when he possesses Shiro , he has typical demon features such as pointy ears, long nails and sharp canine teeth. The possessed 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 their body. In the end of the anime he resembles a small blue flame, which may be his basic and/or true form.
When first introduced, Satan's mannerisms and laughter bordered on that of insanity and he enjoyed it when Rin Okumura 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 who as Satan's offspring bears Satan's blood. He only cares for one goal which is to overtake Assiah at any cost.
In anime filler, it is later revealed that he has a kinder side to him. He was friends with Yuri and it was revealed that Yuri willingly allowed Satan to possess her because she felt sorry for Satan, for everything he touched in Assiah died. Satan's possession later led to Yuri's impregnation.
Satan had possessed and murdered a great number of powerful Exorcists; an event which would later be known as the Blue Night. Around that time, Satan had used one of the bodies he had possessed to rescue Lower Second Class Exorcist Yuri Egin. Yuri later gave birth to Rin and Yukio Okumura. However, only Rin inherited his father's power. He later realizes that he can use Rin to conquer Assiah. In the anime, it is later revealed that Yuri let him impregnate her because she felt sorry for him and that actually he didn't plan to conquer Assiah but uniting Assiah and Gehenna together.
After Astaroth and his army was defeated, Rin told Father Fujimoto that he should not pretend to be his father. Satan took this chance of weakness to posses Fujimoto, since at that point Fujimoto was shocked as to what Rin said. Satan then appeared and opened the Gehenna Gate in order to drag Rin to Gehenna. His plan was successful until Fujimoto took control of his body again, but Satan then stated that it was too late. Anything that gets pulled into the Gehenna Gate cannot escape. With saying that Satan lost control of Fujimoto's body. After that, Rin awakened his powers and destroyed the Gehenna Gate.
Filler arc (Anime Only)
After a backfire of Ernst's plan to destroy Gehenna, Satan releases the Coal Tar to dominate True Cross Academy and speaks out to Yukio, telling him the truth of his and Rin's birth and the connection between him and Yuri Egin. Yuri was Satan's vessel and lover. She taught him about Assiah and how things live on and die. Curious of how to nuture life, Satan impregnanted Yuri. Because she was going to bear Satan's sons, Ernst ordered Yuri to be sacrificed and burnt. To save her, Satan posesses a number of Exorcist to sustain his power but they all died in his flames leading to the Blue Night. He took over Ernst's body and freed Yuri, shortly afterwards she bore Rin and Yukio. She died afterwards and Shiro raised the boys.
Satan waited for them to grow up and for one of them to awaken to their new power. Once Yukio knew the truth, Satan fully posessed him. With a new vessel and enormous Gehenna Gate, Satan uses them to complete his dream: creating a new world by fusing Assiah and Gehenna. He decides to get rid of Rin (since in the beginning, Rin was able to destroy the Gate). Shura and the Exwires intervene and everyone escapes. Satan keeps watch on the Gehenna Gate. We learn in a flashback, that it was Yuri's dream of a world where demons and humans can live together and Satan plans to make her wish come true. Yukio is somewhere inside his own mind when Satan has possessed him so Shura and Shiemi try to call him him but do not prevail. Rin enters, which Satan wants, to duel with him. Rin refuses to fight Yukio and calls to him. Yukio regains his senses and calls for help but Satan uses his body to shoot Rin. Crying in anger, Yukio tries to commit suicide in order to stop Satan but Rin stops him, not wanting to let Yukio do the same that Shiro did. Yukio bursts in tears which expells Satan. However, the Gehenna Gate raises into the skies and Satan plans to finish his and Yuri's dream regardless. He merges with the Gehenna Gate and sends out many more Demons. Yukio and Rin go on the offensive and pierce through the heart of the Gehenna Gate, destroying it and forcing Satan back to Gehenna.
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 posses a human body but only for a moment as everything he touches in the world of Assiah is doomed to destruction.
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 only) 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.
(Anime only) 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 his sons, Satan can control and manipulate blue flames at will. He is able to ignite his victims in blue flame by spontaneous combustion (implied to be 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 Christianity, Satan is the name given to one of the fallen angels who was not only apart of but also started the War in Heaven against God & his angels. Before he became Satan, the rebellious angel was once named Lucifer, which is short for "The Light Bearer" or "The Shining Star". This may not be Satan's actual origin in the manga series but it may be possible.
- Whenever Satan possesses someone the resulting form is actually something close to what people originally believed the devil himself to look like in folklore. The tail, pointed ears and devilish facial features are proof of that.