As part of their plan, demon general Psycho Jenny wipes Satan's memories and replaces them with those of the recently deceased Ryo Asuka in order to infiltrate human society and find their weakness. While living as Ryo, he befriends the timid human boy Akira Fudo, who becomes his best friend, and as Devilman, his greatest foe. Satan wins their war against humanity, but loses everything in the process after realising too late the love he felt for Akira.
When designing Satan's true form, Go Nagai's intention was for Satan to embody the best attributes of men and women. As such, Satan has a shapely, somewhat muscular yet slender curvaceous body with fully developed breasts, but also a penis. They have blond hair and blue eyes, and around their body are twelve golden wings.
As Ryo, Satan has a more masculine, though slender, build. In direct contrast to the always-nude Satan, Ryo dresses rather conservatively, with a preference for long coats, suits, and collared shirts.
Ryo has a very serious, intense, and haunting personality, likely from the threat of the demon apocalypse looming over him. His actions at the Black Sabbath event show that his approach to the war between humans and demons is possibly even more pragmatic than Satan's, instigating violence and dooming an entire group of people to their deaths on the off chance that he or Akira may become devilmen. Despite his initial enthusiasm, the more the war goes on, Ryo becomes convinced that humanity is doomed and becomes apathetic towards their attempts to defeat demonkind, even finding dark humour in humanity's futile attempts to delay the inevitable.
Ryo's only known friend is Akira, who he later develops romantic feelings for. Ryo never tells Akira how he feels, but he is very protective of him, and seems to be the only person Ryo expresses any affection toward, to the point that he flatly dismisses Zennon when he questions him about how these feelings could affect his plans.
Satan has a very strong (albeit misguided) sense of justice, betraying their father God to protect the demon race against God's judgement. This protectiveness does not extend to humanity, which Satan views as a parasite that needs to be eliminated. Ironically, while assuming a human identity, Satan inherits this same sense of justice, but targeted against the demons. Satan only realises when it's too late to turn back that their treatment of humans is no different than God's treatment of demons.
Satan has a wide range of powers, including flight, telepathy, telekinesis, mind control, and firing massive beams of white light from their hands that can cause much widespread destruction. Satan's telekinetic powers are strong enough to blow bodies apart. They can summon a sword when they battle the forces of God in Devilman Lady; here they also demonstrates the ability to split their consciousness in two. Satan presumably has more powers than other angels due to being 'the child of God'.
Ryo Asuka is a normal human with no supernatural abilities, though he is clearly skilled with guns including a sawed-off shotgun used to scare off crowds, and a sniper rifle for long distance attacks (such as when Sirene attacked Akira). Even while human, Satan's great power sometimes manages to slip through, such as during his encounter with Eader. In this supposedly human form, Ryo recovers from wounds and injuries much faster than a regular human, even the kind that should be fatal. Ryo can also sense demons from their psychic signature, allowing them to track down their locations. He is an adequate fighter, able to get his way through a mob of angry party-goers.
Ryo's incredible regenerative abilities aren't limited only to his own injuries; he is capable of healing mortal damage to anyone he chooses, as shown when he was able to make Akira's severed arm and gored abdomen to spontaneously reattach and repair themselves after he was nearly killed by Sirene.
Either form's most dangerous skill however, is their charisma. The beautiful and attractive appearance alone gives off a sense of leadership and their personalities comfort many around them. This allows them to have people do whatever they want by manipulating his desires, vices, and passions. This can range from throwing a large party and getting the party-goers to attract and be possessed by demons, or even cause a nationwide panic by revealing to the public about demons while leaving out crucial information about Devilman. Because of this, Satan commands an entire army of demons with absolute loyalty.
Ryo Asuka is introduced when he finds Miki and Akira being harassed by a small gang led by Dosu-Roku. Ryo tells Akira to come with him before he pulls out a sawed-off shotgun and fires it to drive the gang away. When Akira asks where Ryo got the gun, Ryo only says that he is facing an enemy. Ryo drives them in his father's car back to his home, revealing he illegally modified his father's license. Akira is confused by Ryo's behavior; finally, Ryo dramatically cries out that his father, Professor Asuka, committed suicide.
They finally arrive at the Asuka Residence residence and Ryo tells Akira to prepare himself for what he is about to see. He shows Akira the Demon Mask in Professor Asuka's chilling laboratory. Ryo explains to him that the mask is a fossil his father found during an expedition in Peru. After donning the mask, Ryo convinces Akira to wear it. When Akira puts it on, he is overwhelmed by horrific visions of demons from millions of years ago. After Akira recovers from the nightmarish images, Ryo says that he believes demons still exist and are hibernating in the coldest parts of the world. He then tells Akira the details about his father's suicide, and how his father did it because he failed to fuse with a demon. Since Prof. Asuka could not control the demon, he killed their pets and attempted to murder Ryo. However, traces of humanity in Prof. Asuka reemerged, and he set fire to himself to end his suffering.
Ryo reveals what little facts he's learned about the demons from reading his father's notes, namely that the demons plan to attack Earth and reclaim it in the near future. Ryo tells Akira that he needs his help to fight the demon menace, to which Akira agrees. They prepare to leave and drive down the road before Ryo notices a figure in the distance. When they get closer, the figure reveals herself as the harpy demoness, Illuge. Illuge summons other demons, forcing Asuka and Akira to fight them off. After Prof. Asuka's car is destroyed, they walk back to Ryo's house. Ryo tell Akira the real reason he wanted him around is to fight the demons, but Akira tells him he already knew this and doesn't need to apologize. Ryo lights a cigarette laced with drugs, and the two go into his house through large steel doors at the end of the corridor.
A massive rave iS In full swing. Akira is confused, but then Ryo tells him he is holding a Black Sabbath party. Ryo has planned for either one of them to try and merge with demons, but they must attract the demons first with drugs, sex, mayhem, and blood. Ryo breaks a bottle and slashes at his guests before they grab him and punch him. Akira tries to intervene but gets attacked. Between Ryo's chaotic assault and the two of them getting beaten, enough blood is spilled to smoke out the demons. The demons begin to fuse with the guests, starting with the demoness, Welvath.
A fearful moment passes where neither Ryo nor Akira feel like they are merging with the demons. But suddenly, Akira is possessed and successfully dominates the spirit of a mighty demon named Amon. A brief exchange of dialogue between Akira and the demons transpires, and he then slaughters them. The violence shocks Ryo, who is amazed at Akira's cruelty and the carnage; he even questions if Akira is still the good-natured friend he knew. During the fight, Ryo is knocked out and pinned down by a demon's body. Akira, acting as Devilman, kills the demon and rushes Ryo to the hospital.
The next day, Akira goes to visit his friend in hospital. Ryo asks how Akira is feeling after his transformation. Akira replies that he feels good now compared to how he felt earlier, but that he is suffering from nightmares. Ryo says that the nightmares originate from the demon Akira fused with. He goes on to explain that the creature dominating his nightmares is a demon even stronger than Amon. That night, Ryo awakens after realizing something is amiss. He senses Akira is in danger, so he grabs his gun and leaves the hospital. He encounters a man with a motorcycle, which he steals to chase after Akira. Akira, having just defeated Agwel and Ghelmer, is ambushed by the demoness Sirene, who carries him off into the night sky. Ryo climbs up on a cliff and sees Sirene fast approaching. He shoots her several times in the chest, injuring her and causing her to drop Akira. Sirene counters Ryo's attack by firing off one of her claws, which knocks Ryo unconscious and pins him down.
Akira and Ryo meet several days later in a cafe, and the two talk about humanity's response to the idea of demons and its irrational fears. They eventually agree that humanity's fear largely comes from myths about demons in the days of old, and Akira comes to the decision to destroy all demons.
Late one night, Ryo follows a man named Maegawa home from a restaurant. Waiting till the two of them were in a safe secluded location before he goes up the man and begins the threaten him, attempting to get the man to turn into a demon revealing his true form. The man calls him crazy and insults him, only furthering Ryo's attack as he slashes at his head with a bladed weapon and pierces him through the ear with it. Eventually he begins to transform revealing himself to be the demon Eader, however before Eader can approach and attack he freezes in fear and begins to stutter before Ryo blacks out and falls to the floor. When he awakens it is early morning and engraved on the ground is Eader's shadow. Ryo stumbles along and talks to Akira about what had happened, but Akira only scorns him and says the demon tricked him and got away. Akira then leaves Ryo alone to ponder on the experience.Ryo isn't seen again until the demons devise a mass suicide fusion spree across the world merging with well minded and stable people, causing the two bodies to reject one another and die. Ryo witnesses several of the outbreaks, and it dawns on him that the demons had devised a new strategy and that by merging with random humans it would cause the world to go into mass panic. He runs into Akira and explains to him his theory, to which he agrees. Ryo mentions how he thinks that humanity will begin to turn on each other. Akira doubts this, however on the television they both see a report that the large city of Tsuringrad had been obliterated by a H-Bomb due to the government believing it to have been swamped with non-human entities. Ryo tells Akira that he may now have to combat humanity itself, the very thing he swore to protect as well as the demon menace.
Its not long after the suicide run that the mighty demon lord Zennon announces across the world the existence of the entire demon race and that an attack was imminent. Akira finds Ryo, but he tells Akira not to go out and fight, much to Akira's disbelief. Ryo says to Akira that he needed to stay put as the last hope for humanity. In just under a minute however, Akira witnesses the flames and the carnage around him and attempts to leave. Ryo tries to stop him but Akira hits him and Ryo reluctantly lets him leave to go and fight.
Akira is returned to Ryo after he is knocked out by the demoness Psycho Jenny. Waking up, Akira asks to turn on the TV and sees the carnage that has been caused by the demons with homes in flames across the entirety of Japan and the world. Ryo says that Akira should fight for the honor of humans instead of human lives as he doubted if humanity was going to survive. Akira angered by this throws him against a wall, and tells him that he would not be alone in this fight, believing there are more Devilmen. Suddenly on the television, a news report about Russia came on, the entire country had been engulfed in a glowing light and at seeing this Ryo flips out and cowers in the corner.
After recovering from his previous shock, Ryo views the report on Dr. Rainuma's newly formed Demon Busters. This makes Ryo laugh, as he knows Rainuma's plans will end in disaster. However, it suddenly hits him how many of the things he predicted would happen have come to pass. This makes Ryo anxious. Confused and wishing to be more of an asset to Akira, Ryo rushes home in the hopes of possibly becoming a Devilman, or at least finding more innovative ways to fight the demons.
Ryo finds the Demon Mask but nothing extraordinary happens, revealing the mask is merely plasticine and phosphorus paint; Prof. Asuka's notes are also missing. Soon after, Ryo comes across a photo album and discovers a photo of a child named Ryo Asuka who did not look like him. He flips through and also sees an entry stating Ryo died in a car crash. Upon making this discovery, "Ryo" is bombarded by demons appearing out of the walls and ceiling before he is approached by one who calls herself Psycho Jenny. She restores Ryo's true memories.
After having disappeared for a short length of time, Akira gives up searching for Ryo and decides to go back to the Makimura residence. He watches Ryo being interviewed on television about demons. Akira expects Ryo to dispel all the dangerous misinformation being spread about demons and Devilmen. Shockingly, he betrays Akira by rolling a film of Akira's first transformation and claiming that Akira is a full-fledged demon hiding in a human's body. Afraid of being captured by the Demon Busters, Akira flees from the Makimura household; unknown to him, Akiko and Kozo Makimura are taken in his place.
Hours later during the morning, Akira finds Ryo alone in the wasteland that was once Tokyo. Akira asks Ryo if he regrets his actions, to which Ryo says no and goes on to explain he has no emotions like that anymore. Akira is enraged by this sudden change in Ryo, but Ryo only states that Akira should not interfere with his plans. Ryo says that a new age is on the horizon, one that is just like the age before humans claimed the world. He reveals that Akira merging with Amon was not by chance, but that he orchestrated the events at the Sabbath so that Akira would survive when humanity collapsed. After a brief silence Ryo admits he doesn't want to fight Akira or the Devilmen; Ryo extends a peace offering and asks Akira to stay by his side. Akira stands still and says nothing, having figured out Ryo's true identity as Satan. Ryo/Satan walks off into sunset. As a result of Ryo/Satan's actions, many of Akira's friends and loved ones are killed by vicious mobs, including Tare, Kozo, Akiko, Masa, and Miki. After Miki's death, an enraged Akira vows to kill Ryo/Satan.
The Final Confrontation
Meanwhile, high in the Himalayan Mountains, Zennon congratulates Satan on their plan. Despite this praise, Zennon proceeds to criticize Satan for their one flaw: unexpectedly falling in love with a human (Akira Fudo). Zennon continues to berate Satan about how they caused him to lose some of his greatest soldiers. Satan acknowledges this and reluctantly realizes that Akira will want to seek him out for a final battle. The Devilman and demon armies face off to fight over the wasteland that was once China. A 20 years-long war occurs. Much of the country is further devastated as the two armies clash. Akira loses the fight and his body is severed in half.
Satan lays beside Akira on a large isolated rock in the sea. While Akira gazes at the moon, Satan tells him about their father God and how He created Earth, planted His seed and left. God returned several million years later and noticed some of Earth's creatures had become demons. God judged the demons as imperfect, so He attempted to wipe them out. Satan protested their father's actions and sided with the demons. Satan then led a revolt against God and His angels, and they managed to drive His forces back. Satan voluntarily encased themselves in ice and went into hibernation for two billion years. When they awoke, they found Earth to be entirely unlike how it was when demons ruled. Satan loathed humans for 'stealing' Earth from them and the demons. However, they finally realize that their desire to eliminate humans made them no better than God, who saw demons as weaklings He could destroy.
After this epiphany, Satan begs for forgiveness from Akira. They look over and realize Akira has closed his eyes and entered an 'eternal slumber.' Satan cries over Akira's dead body while God's angels descend to wipe out the earth.
Devilman: One Summer Day
Set not so long after his fight with Sirene, Akira is seen on the beach with Miki and Ryo. Akira asks Ryo wanted to come in the water with him and Miki, Ryo declined and the two went to play in the water. As Akira and Miki frolic around in the sea, Ryo falls asleep and begins to have strange dreams about Akira. Angrily turning and walking over to Miki. Before leaving Ryo calls out to Akira on the floor. As this was happening Akira and Miki are attacked by the Demon Doelamale and Akira has to transform and kill it to save Miki from its jaws.
Devilman: Fallen Angel
In another short story taking place before the final chapters of the manga, Ryo is seen having a dream seeing flashback to his confrontation with God and then finds himself wandering a frozen cave seeing various monstrosities encased in ice. He comes across himself in his true form as it smashes out of the ice causing to suddenly wake covered in sweat.
The story takes place presumably after the defeat of Sirene in the first manga. It begins as Ryo is awakened by strange tremors under the ground before the caterpillar-like demon Oysnas rips out of the ground and attacks him. However Akira bursts in through the window and promptly kills the demon. Akira gloats to himself about his victory before noticing Ryo in the corner in pain he goes over to notice the walls around him beginning to fade before opening the door to find himself in Vienna, Austria just a year before the outbreak of the first World War. Ryo explains that he thinks they are trapped in a timeslip, Ryo suddenly notices a man walking away from them with a strange demonic scent. They follow him to an art studio where he introduces himself as Harnisch with his assistant Adolf painting in the corner. The two realize that the demon was not there but later notice Adolf and Harnisch walking off towards a large gloomy house. Inside they sell a painting of Adolf's past love Stephanie to the head of the local Jewish bank Schultz. Schultz however had kept Stephanie as a personal slave and when Adolf saw him, Schultz became a demon burning her to death giving the fully named Adolf Hitler an immense hate of the Jewish population. However Akira bursts in and kills Schultz, afterwards the time around him begins to warp again as he and Ryo are spun off to a new time.
As they continue on spinning on through time, Ryo tries to give off some reasons as to why this was happening to them unknown to him that this was in fact happening due to his subconscious powers going on the flux. Afterwards they both hear some disembodied laughter in the distance, they turn to see the brooding Court of Hell standing before them in the far off space. Akira investigates and promptly destroys the demons and the building before they continue on their journey.
Next they find themselves in 200 BC sailing towards the island of Samothrace on several Athenian battle ships. The enemy battleships come out to fight them much to the surprise of Ryo, only to see that the Samothracean soldiers were armed with bazooka's, they then proceed to destroy the Athenian ships. Ryo and Akira are taken prisoner alongside several other soldiers and are brought to the top of the isle where a beautiful angel-like being lands. She is revealed to be the demoness Nike and was handing out modern day weaponry to the Samothracean people. When Akira escapes and chases Nike, Ryo somehow teleports away from the ensuing chaos. He finds Akira and Nike on some rocks, it is revealed the Nike was in fact a past lover of Amon's and that she still had feelings for him. However as the two are about to embrace Ryo pops up behind and decapitates her with a sword, killing her instantly.
Afterwards the two find themselves in France shortly before the French Revolution, Ryo discovers that a demon had taken possession over Marie Antoinette. They locate her and both Akira and Ryo are teleported into her subconscious where they find the demon Poirsenus corrupting her thoughts. Akira transforms and fights the creature, Ryo however tells him to be careful in fighting the creature fearing it may affect her actions, his words go unheard however as Akira rips of the demons head after a short battle.
Ryo and Akira end up in a Native American territory before the Battle of Little Horn. They make friends with some of the locals with Ryo catching the eye of a girl. However upon a small trip from horseback, it was ransacked by settlement soldiers including most of the friends they made. Ryo witnesses Akira wipe them all out in anger.
In a short, Satan is revealed to be a fusion between three beings a human male named Lucifer who served as a prince of the ancient world, a woman named Yuria who lived in the garden of paradise with the werewolf Urga, and a pegasus mount owned by Lucifer. During a battle with the demons, the genetic anomalies that gave the demons the ability to fuse was absorbed by the three as Lucifer, Yuria, and the headless pegasus fused together becoming Satan. Empowered and seeing this as the means to create even more powerful beings, Satan appeared to the then human Amon. Satan forcibly merged Amon with his dragon and Urga becoming a demon. The events were seen by Satan's modern counterpart Ryo Asuka in a dream, revealing that he and Akira Fudo had in fact seen memories of Satan and Amon.
- Satan/Ryo does not appear in the original anime, with its version of Zennon being the leader of the demons instead. However, the character Iwao Himura bears a resemblance to Ryo, and is sometimes combined with Ryo in adaptations (such as Devilman Grimoire's Asuka Himura). In a later interview, the writers of the anime state that they would have liked to have included them.
- Asuka is meant to be derived from Akshobhya of the Five Dhyani Buddhas, who represents the wisdom of reflection and consciousness. This refers to Asuka's hidden identity as Satan.
- Satan's appearance post-Devilman Lady is more feminine, including a slightly more round face and lighter eyes.
- In Violence Jack, Satan is once again incarnated as Ryo Asuka, but this time he is in love with Miki Makimura. He is found by Jack as a 'pet' of the Slum King, who amputated Ryo's limbs and forces him to walk on stumps. After regaining his memories and limbs at the conclusion of the manga, Ryo and Slum King transform into Satan and Zennon, and prepare to fight Jack, who equally manages to regain his true form as Devilman, but both antagonists are defeated.
- In Shin Violence Jack, Jack, who is actually Amon, makes a passing reference to an Archangel "that sided with demons" - a clear allusion to Satan. However, Satan does not appear in the story.