Satan is the intersex Angel that rebelled with the Demons against God in the ancient past. They also created the false persona Ryo Asuka, and tricked Amon into Akira's body for their own desires. They appear in the form of a twelve winged angel, similar to the myth of Satan having assumed the form of twelve winged angel at some point in their life.
Ryo is a good friend from Akira Fudo's past. His father committed suicide after becoming possessed by a demon and left Ryo with a terrible legacy. Ryo saves Akira and Miki Makimura from a gang of bullies by pulling a gun on them. He takes Akira to his place and makes him wear the demon fossil that his father uncovered, which reveals what the world was like back when demons ruled. After demons ambush his house, Ryo tells Akira that the only way to beat the demons is to merge with one. Ryo explains that only a pure hearted person has a chance of resisting a demon's will and taking advantage of their powers; if the demon takes over their psyche, they must commit suicide. Both Akira and Ryo cry together in fear, but they clasp hands and swear to protect mankind.
The two go to the Sabbath, a drug- and sex-fueled party, where almost everyone gets possessed after Ryo slashes a woman with a broken wine bottle. Akira gets possessed by Amon, lord of war and one of the strongest demons. Thanks to his pure heart, Akira claims the demon power as his own and becomes Devilman. As Akira slaughters the demons, Ryo watches on in amazement. Somehow, he is injured during the battle and Akira takes him to the hospital. Later, Akira visits Ryo in the hospital and finds Ryo recovering at above average speed, despite not merging with a devil. Ryo and Akira discuss about life, nature, and the fate of the world.
When Akira battles Sirene, Ryo helps him by shooting Sirene with his shotgun before he is struck by her flying claw. After Akira is nearly killed by the combined form of Sirene and Kaim, Ryo heals Akira and reattaches Akira's severed arm, hinting that he has strange powers. Later, when demons attack and possess a lot of people, Ryo realizes that the real Ryo Asuka has been dead for some time and that he is actually Satan. To infiltrate and destroy the human world, Satan was hypnotized by Psycho Jenny and lived as the former Ryo Asuka. (It is even implied that his memories of his relationship with Akira may have been falsified.) Yet Ryo's feelings of friendship for Akira develop into love, even when he uncovers his true identity.
Ryo goes on live television and runs a tape of Akira becoming Devilman. After society is undermined by paranoia and fear - and Miki is murdered as a result of the human's hatred for demons - Akira and Satan have a 20 year war. In the end, with the world destroyed around them, Satan tells Akira that they originally opposed God because of his cruelty toward the demons, whom Satan had grown to love. Satan was wounded in the battle and sealed with the demons in an icy prison for millions of years. When demons are revived and find that humans have taken over, Satan resolves to wipe out humanity and reclaim Earth for its original inhabitants. However, Satan tells Akira they regret their actions, since destroying 'weak' humans is the equivalent of God exterminating 'weak' demons - they have become just as cold as their Father. At the end of their monologue, they look over and see that Akira is dead. Satan weeps over killing the one mortal being who made them love again as God's angels descend to Earth and wipe out the planet once more.
CB Chara Go Nagai WorldEdit
In this OVA, Ryo is more comedic and tends to shoot anyone who offends him with his shotgun (he also seems more openy homicidal). Right after Akira got possessed, Ryo was wounded and didn't get better until AFTER Akira wacked Sirene(who is not only alive and well in this OVA, but has not even met Akira) but both are at full health and Akira has the (demon) power. Ryo and Akira and Miki go to solve the mystery of the world and run into Jinmen on the way to the iceberg. Miki chops his head off and later gets captured by Gelmer(Ryo is awake when this happens but does nothing and even smiles as Miki is taken away). Ryo tried to convince Akira not to save Miki since it's an obvious trap, but Akira flies off to save Miki. When Jinmen says that Akira turned down Ryo, he kicks Jinmen to Akira(showing that he doesn't like being turned down). He is later Picked up by the Mazinger Z Villans and it is revealed to him that he is Satan by Baron Ashura (who thought it was Ryo who used his devil powers to create the Chibi world). Baron Ashura reveals that there are many worlds in the Chibi place and that the Mazinger Z villains will conquer them all(this is where Ryo starts to turn evil(well...just power hungry)). He later tries to kill Miki with a giant laser (for getting in the way of him and Akira) but Akira gets in the way and Sirene attacks. In an attempt to protect Akira, he fires the "Laser" and nails her spinning hands. She comes out of the sky and startles Ryo, causing him to blow a hole in her! Ryo gets freaked out saying it was an accident. Baron Ashura launches a missile attack to capture Sirene and escapes. Ryo tells Baron Ashura to stop harming Akira (because of the missile attack) and puts his gun to Baron Ashura's head. Ashura fights among him/herself(in a comic way that comes to blows) after the male side blames the female side for the attack. When Ryo gets fed up with the fight and points his gun at him/her, Baron Ashura continues to fight and Ryo gives up. After Baron Ashura has punched him/herself out, Ryo attempts to fly the Airship by reading the manual and releases Sirene. He realizes he pushed the wrong button and pushes the right one while being very relieved he didn't push the self destruct button. when they arrive on the island, the doctor asks Sirene to marry him but the only ones on the ship are Baron Ashura/Ryo(who he then orders his robot to kill). Ashura/the doctor fight over who should be killed and Ryo swipes the control stick. when he summons all the mechanical beasts, only one comes out (because the doctor is running out of money from the long battles with Mazinger Z). Ryo gives the villains his "Demonic Express Gold Card"(90009 00071) to help them out if they will help him out(which they do). Ryo reveals that he wants to go to the sun so he can face the creator of the Chibi world. During the battle between Mazinger Z/Sirene/Ghoul, the airship is damaged and needs to be repaired. On the way down, Ryo finds Sirene and she doesn't recognize him...at first. After Realizing who he is, Ryo orders her to marry the Doctor. She refuses and Ryo is about to punish her when Mazinger Z comes in! Ryo knocks Mazinger Z away with a wave of his hand(and an energy blast) and Baron Ashura tells him that the main engine is on fire and they have to land(so Ryo lets Sirene go and promises retribution). Later, they stop to gt repaired at the Getta Robo Saotome lab and pay with Ryo's gold card. Ryo talks with Psycho Jenny about Sirene and how Psycho Jenny looks the same even when Chibi. When Psycho Jenny tells Ryo about Amon's soul being sent to Kaim's body, Ryo laughs says they should be happy together. Later, while watching footage of the sun and seeing a carefree happy face (the one that created the Chibi world) he speaks with Akira (who had snuck onto the Ship) and Akira tries to convince Ryo to give up on returning to the real world and to be happy in the Chibi world. Ryo refuses to have the childlike face rule over him and does not accept the idea of finding parody in himself, so Akira leaves with the knowledge that he will have to fight his best friend. as the airship rises, Ryo sheds a tear. When they get to the sun he fires a black blast to send the corona into another dimension so he can see the real form of the creator of the chibi world. He then brings Mazinger Z and its riders into the front of the ship to show them his victory. When Koji Kabuto gets the arm of Mazinger Z and sucked in the God head, Ryo somehow knows that it went back to the real world and that the God head is a hole in the dimensions. He tries to test the dimension hole by lowering Jinmen in it on a fishing pole but he keeps his eyes shut so he don't see anything(but they did not ask if he heard anything). He plans to send Mazinger Z in there since it has a video camera but Koji Koshiriyoku beams to avoid it. But He says his body is made of photon energy and beam weapons will not work. Akira tackles him from behind and he splits him in half with a blast wave and kills Akira. He acts like Akira is joking and tells him to stop but Akira is already dead. He gets frustrated and tells the God head to do something...And Koji tries the Koshiriyoku energy again(but at full power and on breast fire mode). After a hole is burned through the floor,he uses his power to knock over Mazinger Z and it blows up the ceiling and Mazinger Z falls into the God head... but does not get sucked in and sees scenes of Violence Jack. He takes a peak in the God head and gets startled by a close up of the face of Violence Jack. He tries to explain how the God head creates worlds and says that ideas not good enough to be manga are discarded to the chibi world. He tells the God head to revive Akira or go back in time but he says he can not so Satan(Ryo) tries to battle and ice the God head(even though it may destroy the worlds) since he does not wanna be in a world without Akira. Koji tries to self detonate to save the God head and it blows up the ship! Ryo awakens on the Violence Jack world in the nude with no memories but hates being laughed at and Violence Jack says that without memories or weapons, he can restart.
After Satan killed Devilman, they deeply regretted their actions. Thus they made the Slum King to punish them to make up for what they had done. In Violence Jack, Ryo is in love with Miki Makimura and they are somehow alive and human at the start of the story. They hold jobs at Restaurant King (the Slum King's restaurant) until they run away and get caught having sex by the Slum King's men. They are paraded around in the nude around town and later have their limbs sliced off and pegs put on their stumps so they can walk on all fours. The Slum King imprisons them until Ryo's unseen sister, Honey Asuka, comes to save him. Honey is horrified by his limbless, nude appearance and bombs the Slum King's base.
At the end of the manga, when Honey and her friends get killed, Ryo and Miki help Jack defeat the Slum King and die trying. Later, Ryo returns to being Satan and kills the Slum King for good before resuming their battle with Devilman.
Satan is separated into Lan Asuka (Male Half) and Jun Fudo (Female Half), in order to resurrect Akira Fudo in the physical world and change Lan Asuka (Male Half) to Female to fool Archangel Micheal and in the end Akira joins him/her in the final battle against Archangel Michael and the Army of God, dooming Humanity and Earth.