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.

Desper seems to understand the extent of Ouken's abilities, and warns the Captain of the Knights of the Underworld not to engage him directly. Similarly, Desper warns that Ouken is a natural predator for a swordsman like Bojji, and that the two must not be allowed to fight.

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.

Freezing/Paralysis
Ouken has demonstrated the ability to freeze a person in place with an invisible, distanced attack. The extent and nature of this ability, whether it's psychic or magical, is currently unknown.

Unnatural Regeneration
Ouken's body heals unnaturally quickly, and can even recover from grievous injuries such as having his eyes stabbed by knives. Any blood that has escaped his body is drawn back into it during the healing process.

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).

The Funimation sub of Ousama Ranking has several characters refer to Ouken as "Prince Ouken". This seems to be an inaccurate translation of of the honorific "Ouken-sama" used in Japanese to denote someone of exceptionally-high rank. It's not currently known if Ouken is a prince.