Amaimon's host is a teenage boy that he acquired over 1,000 years ago. He has a spike of hair which is lightly colored compared to the rest of his dark green hair and has long claw-like black fingernails. He wears a jacket that has cross bones as buttons. He also wears green arm warmers and a black and pink short sleeved shirt (same colour as his spike) under his jacket, with rips at the ends. When he is in his Demon form and at full power, he has longer ears, two horns on his head, an alligator-like tail, and muscular arms like his familiar, Behemoth, all of which are green with red markings. He stands at 5'8"; an average height.
After Rin defeats him in the anime, he later appears in the form of a light-green hamster with a white underbelly. In this form he retains the spike on top of his head, has pink eyes, and still possesses the ability to speak. He is always seen with Mephisto in this form.
Amaimon appears to be calm and collected, but holds his power in a very high regard, very brash and, overall, quite carefree. However, he shows great anger and annoyance when someone ruins his fun, like when Mephisto intervened in his fight with Rin in the Chuchi forest. Amaimon also hates losing, as after he lost to Rin, during the hearing against Mephisto (in anime only), to ensure his victory against the former, Amaimon assumed his Demon form. He is very playful and doesn't take his opponents seriously in battle, which suggests that Amaimon is either highly arrogant, or hideously skilled in combat. He has also been shown to care for his familiar, Behemoth, and showed great distress when Shura killed it (anime only). He does not tolerate being laughed at and does not like to be ignored. He was sent back to Gehenna with his father Satan (anime-only).
Amaimon arrives atop a skyscraper that is under construction and apologizes to Mephisto for being late. Mephisto then comments that it has been awhile and calls him the King of Earth. Amaimon acknowledges that it has been sometime before Mephisto immediately asks him if he has an answer from their father. Amaimon reports that their father will accept his proposal. Mephisto is very pleased to hear this and promptly gives Amaimon a message to give to their father upon his return home. Amaimon acknowledges him and then asks if this means he will not be coming home anytime soon. Mephisto does not answer and instead tells him that he does not want to make their father wait.
Amaimon steals Kurikara from Rin while perched atop a statue of his brother, Mephisto in Mepphyland. Rin is startled and immediately asks who he is an demands he gives his sword back. Amaimon tells Rin that he is one of the demon kings and that he is kind of like an older brother of his. Rin then falls silent in shock as Amaimon plays with Kurikara. As he unsheathes the sword, Rin's flames appear.Rin yells at him to stop as he ponders the nature of the sword and it's powers. Rin then jumps at Amaimon in an attempt to recover it, but he Amaimon dodges his attack and it instead decapitates Mephistos statue.
Rins asks what his objective is and he plainly states that he came to play. He then begins to taunt Rin by clapping his hands and telling him to come towards the sound. Rin then dashes towards him.  Amaimon quickly dodges once again and begins to taunt Rin once more from atop a roller coaster. As Rin dashes towards him, he counter attacks destroying a large part of the roller coaster and sending Rin crashing towards the ground.  Amaimon then begins to attack Rin as they fall to the ground, saying that he is a let down and that he does not understand why his brother and father are entranced by him.
As he sits atop Rin on the ground he begins to speak before Rin grabs him and throws him into a neighboring building. Clearly enraged, Rin presses his attack which greatly amuses Amaimon.  After Rin becomes distracted and saves Shiemi, regaining control over his flames, Amaimon kicks him off and asks him if he is already done and then says how boring he is. He then begins to ponder breaking Kurikara before Yamada intervenes and attacks him. He asks for Yamada's identity as he questions how he got into the academy grounds and if Mephisto helped him. Amaimon calls Yamada a hindrance and then backs down, throws Kurikara towards Rin and runs away.  He is then later seen in Mephistos office and comments that it's sad that Shura does not trust him, after she delivers her report about Rin.
Amaimon is annoyed that Shura notices him and Mephisto observing what is transpiring in the forest. Mephisto asks him if he has finshed greeting the master of the forest and Amaimon comments that after killing many of it's brethren that it will cooperate with them. He then tells him that Shura is hiding Kurikara and then wonders how they will be able to unleash Rin's power. Mephisto tells him he is to not move unless he tells him to and it is then that he notices that Amaimon is ignoring him.
After everyone returns to the campsite after a successful task, Amaimon then makes his presence known. Jumping in with Behemoth, he then orders his familiar to attack. Shura then activates a magical circle she drew blasting Amaimon back and erecting a barrier in the process. Amaimon lands in a tree next to Mephisto and is clearly angered, remaking that he wants to kill one of them. Mephisto tells him that if he does he will slaughter him and Amaimon promises to control himself.
Shiemi Moriyama under Amaimons control due to Chuchi larva, then walks outside of Shuras protective barrier and into Amaimons hands. He tells the group that she acquired the parasites during her encounter with the Chuchi and that she will now obey all of his commands as a result before jumping away with her in his arms. Rin catches up with him and asks him what he intends to do with Shiemi. Amaimon thinks for a moment and then says that he will make her his bride before beginning to recite vows. As he seemingly moves in to hurt Shiemi, Rin dashes at him and hits him on the head with Kurikara in it's scabbard telling him not to screw around. Amaimon then exclaims that that is his line and knocks Rin back with an attack. He tells him his sword is useless since he won't use it and questions why he will not unsheathe it.
Amaimon then asks Rin about how important Shiemi is to him. Rin tells him to drop dead. Amaimon then says that she is now less useful to him and that perhaps he will take one of her eyes to give to his cousin. This enrages Rin as Ryuji and the other begin to shoot flares at Amaimon, much to his surprise. Eventually one hits his hair, fraying it. Renzo comments that it looks like broccoli and then Amaimon goes on the attack against the group.
As he takes on the group one by one, he tells them not to laugh at him. Ryuji then begins to talk to Rin and question him. Amaimon becomes angry and picks him up by his throat and tells him he does not like being ignored. Rin then screams for him to stop and eventually unsheathes his sword, revealing his true form. Amaimon is ecstatic and then immediately turns his attention to Rin and goes on the attack.
When invited by Mephisto to join him for dinner, Rin's mind is transferred to Gehenna. Unleashing Amaimon from his captivity, the demon king of earth states he won't lose again and reveals his heart. With his true power, Amaimon defeats Rin with a single blow, but the stress on his mortal body causes him to collapse. Afterward, Mephisto returns Rin and his consciousnesses back to Assiah and returns Amaimon to his prison.
After the events of the Aomori arc, Mephisto released Amaimon from captivity. He later encounters the Exwires whilst wearing a True Cross Academy uniform. It is revealed that Mephisto enrolled him in day classes under the name Ambrosius Faust, claiming Amaimon to be his "nephew" just transferring in.
Having heard the name of Ambrosius during the early encounter with him at the Academy, Ryuji recalls the name after reading it in a case file as the code name of a successful patient at Section 13 during his and Lewin's investigation into the Blue Night. A photo of Amaimon's possessed human vessel is included in the case file, which looks very similar to Amaimon's current appearance less the demonic features like his spike.
- In Demonology, Amaimon is the only one who can control Asmodeous, the Demon of Lust.
- Amaimon is also the Prince of Hell in Demonology.
- (To himself after talking with Mephisto Pheles) "Both brother and father have become entranced... How long will it be until Rin joins them? It'd be good to kill a little time..."
- (To Rin Okumura) "Who...? Ah, yes. I'm Amaimon, a Demon king. I'm kind of like your big brother. Nice to meet you."
- (To Shiemi Moriyama) "Let's see...Yeah! I'll make her my wife. Do you promise to be faithful to me in good times and bad? In sickness and in health? Will you love and honor me, all the days of your life? Then seal this covenant with a kiss...."
- (After being defeated by Rin Okumura) "No way... I'm...the King of Earth... I can't lose! Brother!"