After feeling depressed over killing Akira Fudo, Satan created the Slum King to punish himself. The Slum King keeps Ryo Asuka and Miki Makimura, who's arms/legs have been sliced off, as his pets. He is the same height as Jack and has no skin, but instead wears samurai armor. He is the ruler of earthquake-devastated Kanto and has two children, Zubaban and Jannu. He battles against Jack multiple times and finally loses to Jack due to interference from Ryo Asuka and Miki Makimura, which results in a 3 page beheading. He is alive and well after his brutal death in the next chapter and even beheadsZubaban with a one hit kill but was ultimately killed by newly-revived Satan when Satan returns him and Zubaban's head to their true form as Zenon. His face is not revealed until Jack removes his mask in battle.The Slum King‘s face revealed
He is strong but that just comes with being tall and buff. He also is very sensitive as he has no skin so being out of his armor stuns him. He also has wicked sharp katana skills and his armor can withstand deathrays (such as from Iron Kaiser) and can can cause almost any blade used on him to break (Despite the item user not suffering any recoil or backlash from using enough force to snap a metal item). He also is most of Zenon in another form.
In the Violence Jack Harlem Bomber OVA he is just a warlord who has taken over Kanto with an iron fist who was offended that Violence Jack was stronger than him so he sends his (most possibly weaker) minions to take out Jack. He is seen again in the 3rd OVA as a nonspeaking cameo at the end that was contacter by Dante's blonde man but that is as far as the story goes in the anime as there are only 3 OVA's.
Slum King's name may have been derived from the British rock band King of the Slums.
Given how the Slum King keeps Miki and Ryo as limbless sex slaves, it is implied that he may have some acrotomophiliac (amputee fetish) tendencies. However, this could possibly just be just a punishment assigned to Miki and Ryo or it could be an allusion to the Yakuza disclipinary ritual known as Yubitsume, a ritual that involved cutting away a person's finger or other part of their body as punishment for an offense in a Yakuza group.