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).
True Cross Academy arc
Here, he is seen having discussions with Mephisto, and tested Rin in an amusement park and in the forest.
Training Camp arc
Amaimon attacks the Exwires to get to Rin, who he later fights head-to-head. However, he is stopped by Mephisto.
True Cross Academy Festival arc
- (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!"