Setsuo Miyamoto, a former firefighter turned mentally disturbed killer, murdered four innocents for amusement. Feigning insanity during his trial, he was acquitted only to undergo spontaneous human combustion in the courtroom, transforming into a sentient Infernal—a rarity enabled by his fierce attachment to life. Instantly, he killed two mocking spectators, executed the judge who freed him, took his lawyer hostage, and vowed to slaughter the city’s population, rampaging indiscriminately. During his spree, he confronted Special Fire Force Company 8 members Shinra Kusakabe and Arthur Boyle. He targeted a distressed woman and a young boy, musing on destroying youth’s purity, and hurled a car at civilians—intercepted by Shinra. As an Infernal, he possessed dark blue skin, a muscular build, and a vertical antenna-like structure above his mouth, flames flickering behind it where his cheek was absent. His prison handcuffs morphed into fiery, malleable weapons. Company 8 scarred his face with a gunshot and Shinra severed his right forearm. Captured by Princess Hibana and Company 5, he endured violent experimentation: forcibly drugged for enhanced strength and fitted with an armament prosthesis replacing his lost limb. This prosthesis concentrated his flames into a devastating directional blast. Company 5 clad him in their uniform trousers and a white head-cloak marked "5." Later, alongside Company 5’s Conehead, he battled Arthur Boyle, mocking his weakness and initially overpowering him. Arthur realized he fought with his sword Excalibur in his non-dominant hand; switching hands, he bisected Setsuo with "Violet Flash," killing him. After the second Great Cataclysm, Shinra Kusakabe’s time manipulation reset reality, briefly reviving Setsuo as an Infernal once more. Setsuo exhibited relentless sadism and self-interest. As a human, he disregarded others’ lives; as an Infernal, he savored painful killings while displaying tactical cunning and self-preservation. He hypocritically branded Fire Force members murderers while slaughtering humans himself, rationalizing his violence by claiming his firefighter rescues granted him ownership over lives. He deemed himself superior to mindless Infernals due to his retained consciousness. His pyrokinetic abilities included generating head and wrist flames for flight and fireballs, manipulating fiery handcuffs offensively, and punching through reinforced materials. Company 5’s enhancements amplified his strength and focused his flames into concentrated blasts via his prosthetic arm.