It isn't known how long the Demon King has reigned - there has been no mention of predecessors so far; so he might be one of the few Immortals of the Innworld. Otherwise, Demon King might also be just the title and there are regular inheritances in some way.
Two famous champions of the Demon King, a [Mage] Demon and an [Archer] Demon both over level 40, invaded Paranfer with a small army of Fearless in the same year that the Earthworlders were summoned. Tom killed the [Archer], while the [Mage] could escape. To get into the capital of the kingdom, the Demon King conspired with the Fool.
Powers and Abilities
His classes remain unknown, he might or might not be an actual [King].