The Demon king (魔王) Is a character from Live A Live and is from the scenario King of Demons. Not much is known about the Demon King as he was defeated and killed by Hash and Uranus years before the start of the story. Before he died he crafted seven statues in a chamber where he could send himself to various timelines and create various incarnations of himself so he could continue living. His statues can also be used by anyone who is to become his successor to do the same thing such as when Oersted made use of them when he became Odio.
The Demon King never appears in the flesh since he was killed years ago however it can be seen from presumably a statue of him on the mountain that he had a sort of dark knight like appearance complete with a helmet that covered his whole face and a cape.