Zennon

Zenon (Devil King, or Great Demon King Zenon) is a Demon from Demon Lord Dante and Devilman. He is often portrayed as powerful and big. He has a different appearance in Demon Lord Dante than he does in Devilman. In the Devilman TV Series his name is changed to Zannin　and is referred to as Demon Beast Zenon in the English dub of Demon Lord Dante.

Demon Lord Dante
In here he is a demon who sold his soul to God in exchange for power. He appears to Ryo Utsugi claiming to be king Herod the man who betrayed Jesus,in a napalm fire after his demon transformation and attack on the city to make him give up on defeating God. However Ryo isn't aware of the past of Dante at this time. After a conversation about Ryo being Dante and his heart being Dante's heart he attacks Ryo with his 100 demons. He melts his demon warriors on Ryo to trap him in their goo but somehow Ryo wasn't in the demon goo (despite it forming his exact shape) and he rips Zenon in half from behind. As he was dying Zenon told Ryo about how he wanted God to be defeated and to raise a demon army to beat God as well as saying reference about Maoh Satan, Adam and Eve before dying offscreen.

Devilman
In here he seems to lead the demons but it turns out he works for Satan. In Devilman Manga Volume 2, Sirene asks him for the power to beat Devilman so he sends Kaim and some other demons to aid her. In dialogue with Satan, he fills in the missing plot of the Devilman manga near the end of Devilman Manga Volume 5. He is seen as Satan's giant dragon/hydra mount in the final battle (but its not revealed that its him until the last chapter of Violence Jack).

Devil Lady
He appears in the manga later in the story. He's frozen at the lowest level of hell and is said to be Satan. Akira says that Zenon and Satan are one the same and can separate to it's original form. At the moment one of Zenon's face is missing and stating Ryo/Lucifer is in the real world.

Trivia

 * Zenon's appearance bears some similarity to the Divine Comedy Satan despite that fact that Satan also exist in the series until in Devil Lady its revealed that Zenon and Satan are the same being and can separate.