Satan is the secondary protagonist and later the antagonist of the Devilman OVA series. Going by the human name Ryo Asuka, he helps his friend Akira Fudo gain the powers of a demon to become Devilman.
As Ryo, he was a handsome and well built boy in his late teens, he had blonde neck length hair and piercing blue eyes. He wore stylish clothing consisting of a striped blue sweater, beige pants and matching long rain jacket as well as a pair of brown sunglasses. In their real form as an Angel, they had twelve white wings, six large ones on his back, two smaller wings on their feet, two on their head and two more on their arms. In this form Satan was intersex.
Satan possessed a great and vast range of powers, even as Ryo he was an expert survivalist and fighter. He proficient in long range marksmanship, being able to fire at Sirene with a sniper rifle at several miles away. This was perhaps best shown in that he survived feats that would normally have killed a regular human, such as having been beaten by demons. They were able to fly with their wings and could fire powerful bolts of light from their hands capable of causing widespread destruction. They also had various psychic abilities being able to manipulate those around them.
Satan had a misguided sense of justice, they hated their own race, the Angels, for trying to wipe out the demon race simply due to finding them impure. However after they awoke from their hibernation to see what humans had done to the world, they became like those they hated, and attempted to wipe out humanity, only realizing their mistake when it was far too late. He also cared deeply for his friend Akira Fudo.
Devilman: The Birth
Satan first appears during the OVA's prologue, inside a massive demon, controlling it as they would a battleship as it neared a cluster of angels during the Mezozoic era.
They then appears as Ryo Asuka during the twentieth century. Ryo meets up with Akira and Miki Makimura while the two of them were walking home from school. He confronts Akira, asking his old friend to accompany him back to his father's mansion in the woods. At first Miki wishes to tag along but Ryo pulls a large knife at her, requesting that he only needed Akira. Akira agrees and the two ride through town alone, with Ryo driving the car in a frantic fashion. Ryo reveals to Akira that his father had committed suicide by setting himself ablaze with gasoline. He continues by saying that prior to the suicide his father had shown murderous intents, having killed Ryo's pet canary as well as decapitating his dog John. More still, doctor Asuka even tried to stab Ryo to death while in his sleep. As he continues driving Ryo informs Akira that his father had left him "a gruesome inheritance" that was linked somehow to his bizarre change in personality.
The two finally make it to Ryo's home. Ryo reminds Akira that his father had been an archaeologist researching Mayan ruins until he had accidentally stumbled into an underground cave, wherein he had made a startling find. Inside a dark room Ryo reveals said discovery, a large, glowing skull shaped much like a classical rendition of a demon. At first, Akira believes it to be a Mayan artifact, but Ryo corrects him by saying that the skull predated the Ice Age. He also states that by wearing the skull over your head it could project images of the Mezozoic era, once ruled by demons. Akira objects to putting it on, but Ryo coaxes him to do so if he truly wanted to discover why Ryo's father had committed suicide. After putting the mask on, Akira's mind is bombarded with imagery of the Mezozoic, a period in Earth's history ruled by devils, grotesque and ultra-violent life-forms with the power to fuse with other creatures, as well as inanimate objects. Ryo continues to explain that these demons were not an extinct species in the same way as the dinosaurs, but rather they had survived by means of suspended animation, frozen in ice during the Ice Age. He brings up the final chapter of the Inferno Book in Dante Alighieri's "The Divine Comedy", specifically the scene wherein the three-headed demon Lucifer is frozen in ice as a prime example of humanity having been made aware of the devils' existence. He then deduces that the devils have begun freeing themselves from hibernation and are likely planning an invasion on humanity out of spite for them having taken over the world in their absence. Despite Ryo's claims that the human race could defend itself with all its greatest armaments, Asuka counters this by stating that there had to be devils whose powers could rival that of human weaponry, including the atom bomb. However, Ryo also briefly brings up the one method needed to stop the upcoming devil invasion. But, his reveal is cut short after the demons Texsch and Gardalza break into the mansion, bent on killing the two teenagers. Fortunately, Ryo out a shotgun and starts firing at the demons, enabling the two of them to successfully make it back to the car. After a brief scuffle wherein Akira and Ryo manage to kill another demon waiting outside and one that had taken possession of the vehicle, they return to the mansion. In the main hallway Ryo opens up a hidden control box that opens up an elevator leading to an underground complex where they make refuge away from Gardalza.
Ryo leads Akira through a long, dimly lit hallway, until they reach a door at the end. Past that door stood the only means through which they could stop the demons. However, Ryo warns Akira that the only way for them to defeat a demon is to abandon their humanity and become themselves fused with a demon in order to take possession of their powers, the process being permanent. Because of their pure hearts they could overcome demon possession and instead take possession of a demon, the result of which being a "devilman." Despite his obvious fear Akira agrees to the plan. As he opens the door, Ryo reveals a loud and crowded nightclub of all things. He explains to Akira that demons could fuse with human beings only after they have given up all rational thought since demons where themselves beings devoid of rationality, much like animals. To that end, he had personally gathered all of the nightclub patrons together to participate in a Black Sabbath, a ceremony where they would consume drugs, alcohol and dance until they became irrational enough to be possessed by demons. During the Black Sabbath, Ryo started a fight with the patrons in order to further the demons' summoning. This prompted several of the patrons to fight back, resulting in their possession by the demons: Wagreb, Welvath, Vegard, Vishvalic, Najuwarle, Kagol, Jurad, Esunotto, Magoad, the Unnamed Bug Eyed Demon and Unnamed Short Demon. As a result, the demons begin eating and murdering the remaining humans, including Ryo and Akira. This in turn causes Akira to panic which leads to his possession at the hands of Amon. Now having transformed into a devilman, Akira begins slaughtering the demons one at a time. And although he his successful with all demons having been killed, Ryo is left mortally wounded, much to Akira's despair.
Devilman: The Demon Bird
Akira managed to deliver Ryo to a hospital, where he was treated for his injures, although his rapid healing did stir some awe from the hospital staff. Akira pays a visit to Ryo in order to check up on him, as well as to reveal his change in behavior after fusing with Amon. Ryo assured him that his mood shifts were to be expected. He then proceeds to explain to him that the demons' reappearance on Earth was likely nature's way of balancing out the food-chain. Because human beings were basically without a natural predator at their current state demons would likely assume the role of their predator. However, he also assured Akira that devilmen where the demons' natural predator, so there was no need to needlessly worry about fighting them. Ryo also reveals to Akira that he had developed psychic powers, now being able to detect the demons' presence.
Later that night, Ryo sneaks out of the hospital and drives after the winged demon Sirene, who had captured Akira and prevented him from transforming into his Devilman form. Ryo climbs to the top of a high-rise and pulls out a sniper-riffle with which he begins shooting at Sirene until she lets go of Akira, now free to assume Devilman form. But Ryo is also knocked out by Sirene's detachable left hand. Ryo does eventually wake up and once again chases after the two. He finds Akira mortally wounded in the woods outside Tokyo, his left arm having been chopped off, but nevertheless victorious against Sirene and several other demons, including Kaim who had willingly fused with Sirene in order to give her a fighting chance. The following morning, Akira wakes up, fully healed and his arm now reattached. Ryo explained that he had simply placed Akira's arm back and it fused on its own.