Caver "Demon King"

About

Caver is the Demon King, an existence created by the world in order to kill the hero. Since he is a being that can only be killed by a hero, he has mastered all weapons techniques and arts due to his long lifespan.

The appearance of the Demon King is based on humans, the only difference in appearance being the black sclera. He has short black hair, but it's said that in the past his hair was originally long. He recruited Deon Hart - the person he misheard as Demon Arut - to become the specialized Zero Corp's Captain after witnessing Deon "kill" the Hero.

(Source: Namuwiki)