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.
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.