The Supreme King is the wielder of The Gentle Darkness and the malicious persona of Juudai Yuuki that possessed him while he was in the second alternate dimension searching for Johan Andersen. He serves as the second main antagonist of season three after Brron and his Dark World Army are defeated, starting from episode 136 until his defeat in episode 143.
According to Daichi Misawa (and later confirmed by Yubel him/herself), it was Yubel who orchestrated the possession of the King over Juudai, for the intent of ruling over the united twelve dimensions together.