Zennon is a towering beast but is small compared to his other counterparts. Zennon has thick black fur covering his body and bat-like purple wings under his arms. He has three heads, the left one being a blonde haired skull, the central being the most bestial with large horns and three eyes and the third head resembling some kind of feminine human appearance with large yellow eyes and long green hair. On its chest is a large mouth lined with sharp teeth.
Zennon is capable of flight with its massive wings. Unlike other demons, Zennon controls three separate bodies.
Zennon is a fierce demon and a brutal leader who like most demons was violent, cruel, and only saw value in how they aided him in battle. As such he follows only the strongest being while allowing anyone strong enough into his ranks. But he is also willing to take matters into his own hands if he spots a weakness, commanding demons to kill Satan before he fully awakens after recognizing the angel's attraction towards Akira Fudo.
Zennon is first seen as his three human hosts a meeting as demons had been getting killed in a certain area. Zennon commands the demons to narrow down the area to find the perpetrator.
Zennon would appear in his full demon form when he confronts Akira with the rest of his army. He and his army attacked Akira while he was in his Devilman form, with Zennon trying to communicate with Amon to get him to take over. After beating Akira down, Psycho Jenny calls Zennon off, which Zennon opposes, but misses his chance to continue fighting when Miko saves Akira in her Devilman form.
Zennon later fights Akira one-on-one, still urging Amon to take over Akira's body. He dies in this battle after being severely wounded.
Zennon was planning to overthrow Satan, as his men blow up the apartment that Satan was staying in.
- Zennon's three split form is similar to ideas seen in Devilman Lady and Devilman Saga, where the great demon is built up of a combination of other devils.