
The demon Amon is a major staple character in the Devilman franchise. He rarely appears in the original manga but has a looming presence throughout. His core character arc is that he is the demon that possessed Akira Fudo, transforming him into Devilman.
His character is rarely described other than being a mighty demon warrior but apparently also cared for Sirene. In later stories such as Shin or Neo Devilman his character is expanded. But it is not until the manga, Demon Knight that he is given an origin and a full appearance.
Amon is by no means good and would at times kill someone with little retaliation; however, he was not evil. He didn't kill for no reason and would only do so when attacked or if someone stood in his way. He did show compassion for some people, the most notable was Sirene but in spite of what Sirene says about their relationship, Amon did not appear to have romantic interest in her only seeing Sirene as a close friend. Amon seemed to have a lover in the form of Nike, where his feelings temporarily surfaced from Akira Fudo, also implying that Akira's flirting and teasing with Miki was somewhat influenced by Amon given Miki's similarity to Nike.
Amon for various reasons bears a strong grudge against the fallen archangel Satan, notably for putting him in Akira's body and want payback against the Angel. However Amon does appear to know fear as Akira saw an image of Zennon through a nightmare in connection with Amon. In his original human form in Demon Knight, Amon was loyal to both his prince Lucifer and to God alongside Sirene. But upon being betrayed by both parties he adopted his more ruthless persona seen in later years.