Satan is the hermaphroditic Angel that rebelled with the Demons against God in the ancient past. S/he also created the false persona Ryo Asuka, and tricked Amon into Akira's body for his/her own desires. S/he appears 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 his/her life.
Ryo is a good friend from Akira Fudo's past. His father had killed himself after becoming possessed by a demon and left Ryo with a terrible legacy. Ryo saves Akira and Miki Makimura from bullies by pulling a gun on them. He takes Akira to his place and shows him what the world was like back when demons ruled the earth.
After an attack by several demons, Ryo tells Akira that the only way to beat a demon is to be a demon. Ryo tells Akira that a pure hearted person can resist the demons will and use the demon powers as their own, but if the demon takes over then they will have to commit suicide.
They go to a nightclub/demon ritual and almost everyone gets possessed. Akira freaks out and gets possessed by one of the mightiest demons, Amon, the Lord of War. Thanks to his pure heart Akira claims the demon power as his own and becomes Devilman! As Akira kills the other demons, Ryo is amazed/surprised by how Akira acts.
Somehow, Ryo is injured in the battle and Akira takes him to the hospital where Ryo recovers. Later, Akira visits Ryo in the hospital and finds him recovering at above average speed. Ryo didn't merge with a Demon but he and Akira have a talk about life and nature.
Later, when Akira is fighting Sirene, Ryo helps by shooting Sirene with his trusty shotgun before getting hit by her flying claw. After Akira is nearly killed by the combined form of Sirene/Kaim, Ryo heals Akira and reattaches Akira's ripped off arm, showing he has some strange powers. Later, when demons are attacking and possessing alot of people, Ryo finds out that the real Ryo Asuka has been dead for some time and he is really Satan. To infiltrate the human world, Satan had been Hypnotized by Psycho Jenny and went out as Ryo Asuka. but Ryo's feeling of friendship for Akira turned to love for Satan and he had developed love for Akira.
Ryo goes on TV and shows a tape of Akira being possessed and becoming Devilman. After the world suffer from fear and paranoia and Miki is killed, Akira and Satan have a 20 year war that destroys the earth. In the end, Satan tells Akira that God saw the demons had evolved into what he had considered ugly, so Satan rebelled against his/her father (God) and the demons were sealed in ice by and with Satan.
When the demons returned, humans had taken over so s/he tried to wipe them out. Satan tells Akira-Kun s/he regrets his/her actions since his/her wiping out humans was just as bad as what God tried to do to the demons and looks over to Akira to finds that he is dead (all that is left of him is the waist up) and Satan cries over kiling Akira.
CB Chara Go Nagai World
In this OVA, Ryo is more comic and tends to shoot anyone who offends him with his shotgun (he also seems more 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.
Devilman FilmInformation Needed
After Satan killed Devilman, s/he regretted this action. So s/he made the Slum King to punish him/her to make up for what s/he had done. In Violence Jack, Ryo is in love with Miki Makimura and they are somehow alive and human at the start of Violence Jack.
They had jobs in the Restaurant King (the Slum King's restaurant) until they ran away and got caught having sex, by the Slum Kings men. They were paraded around in the nude around town and later had their arms/legs sliced off and pegs put on their stumps so they could walk. The Slum King mistreated/abused them and when Ryo's until then unseen sister Honey Asuka, comes to save him, she is horrified by his limbless/nude appearance and later 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 his/her fight with Devilman.
In here Satan separated to two halves Lan Asuka(Male Half) and Jun Fudo(Female Half) in order to resurrect Akira Fudo in the physical world and change Lan Asauka(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.