Ouken

Ouken is the former Commander of the Knights of the Underworld, and such a fierce warrior that he's known as the Sword King of the Underworld.

He was imprisoned in the Underworld until being freed by Miranjo.

Appearance
Ouken is a somewhat tall, thin man who wears head-to-toe plate armor with shaggy black pelts affixed to the back. Rather than a normal visor, his helmet features an intricately-wrought face with a strange, slightly puckered mouth, completely black eyes, and long devil horns protruding from its top.

Personality
Ouken appears to be deeply disturbed, and seems to relish acts of violence so much that he trembles in anticipation of doing them. Moreover, after attacking an opponent, Ouken likes to crouch down and observe them as they die, refusing to deal a killing blow.

Though he has yet to speak, Ouken demonstrates a very upsetting personality, and seems to enjoy inflicting pain and misery on others.

Plot
Ouken is one of the Underworld prisoners freed by Miranjo to conquer the Bosse Kingdom.

While all the prisoners are notorious in their own right, those familiar with Ouken seem to openly fear him, and it's suggested that he is, by far, the most dangerous of the group. He easily dispatched Kingbo, another prisoner who was a former King and a very capable warrior, in a brief altercation.

Master Swordsman
Ouken is a master swordsman, and was considered the pinnacle of swordsmanship in the Underworld. His title, "Sword King of the Underworld," suggests he is at least as powerful as a king, and may have been a literal king himself.

Trivia
Ouken's name is a romanization of the Japanese words 王 (Ō, meaning king) and 剣 (Ken, meaning sword). Because of this it's unclear if Ouken is actually the character's name, and it may potentially be his title. The implications of this are unclear, as it may mean Ouken was once a King himself.

Currently it's assumed that Ouken is his personal name, as the naming conventions in Ousama Ranking tend to be very literal (King Boss, Kage of the Shadow Clan, etc).