Medoin is described as a man with two horns. He is tall and has a muscular body, with a goatee in his chin.
2,000 years ago, Anos created the Seven Elder Demon Emperors from his own blood. After Anos's sacrifice, the Seven Elder Demon Emperors co-operated with the Hero Kanon to create the fictional Demon King Avos Dilhevia as a scapegoat for the human's revenge in Anos' place. Kanon then fused six of his roots with 6 out of the 7 demon elder emperors, including Medoin, and erased their memories so that if they came in contact with Anos when he had reincarnated, he will not know about Kanon's plan because they couldn't lie to him.
During the academy exchange with Hero Academy, the remaining Seven Elder Demon Emperors disappeared from Dilhaide and were sighted entering the human royal capital Gairadite. Though at first, they were hiding, Medoin, Zorro Angart, and Eldora Zaia finally made their move by attacking the temple holding the legendary holy sword of the Hero Kanon. By the time Anos arrived, the trio had already succeeded, as Avos Dilhevia was able to draw out Evansmana. The ensuing confrontation between Anos and Avos doesn't last long, and Avos leaves with the trio to broadcast a declaration of war all over Dilhaide against the human kingdom.
Following the declaration, the demon army settled on the Eiyan Hill from the Dilhaide side of the Tora Forest, creating many demon king castles as their headquarters. When Anos attacked the army, Medoin and the two other Elder Demon Emperors flew out to intercept him, but were stopped by Melheis Boran and the other Elder Demon Emperors allied with Anos. Once Anos made it to the main castle, he was informed by Melheis that Medoin and the others were captured, but that the roots from the people who controlled them escaped.
Unnamed black hole attack: Medoin summons a black sphere that sucks in all people and attacks in range.