Marugori

In human form Marugori was a brawny young man, thick across the chest with powerfully built arms, a shaved scalp, and a hard, piercing gaze. His outfit was nothing fancier than a tank top and ordinary jeans, all of which ripped away the moment he changed. The transformation sent his veins swelling and his muscles ballooning as he grew in scale, leaving a giant that looked like a skinless human giving off steam. Pale, bone-like plating shielded his enormous muscle, with much of the sinew, tendons, and skeleton laid bare beneath it, and two tubes sprouted from his chest to curl over his shoulders toward his back.

Direct and uncomplicated, Marugori stayed devoted to his brother while a hunger to grow ever stronger drove him relentlessly. For all the savage ruin and countless deaths he brought about, the loss of that same brother struck him with real sorrow and rage.

It all started in a lab belonging to Fukegao, the scientist who happened to be Marugori's older brother. Fukegao passed him the finished Biceps Brachii King steroid, and Marugori gulped it down, hungry for the power he had always craved. Pain tore through him as his body shot upward, punching through the ceiling and warping him into a monstrous giant. He set his brother on one shoulder and tramped off toward D-City, musing on his lifelong wish to become the world's strongest man, then tested his might by leveling whole rows of buildings with a swing of his arm while a delighted Fukegao egged him onward.

Heading next for B-City, Fukegao caught sight of someone perched on the far shoulder and screamed at him to kill the trespasser. Marugori struck the wrong shoulder and flattened his own brother instead. Fury took over: he snatched up Saitama, dashed him to the ground, and pounded him again and again into a vast crater. Sure he had won, he stopped to mourn, only for Saitama to spring from the pit and drive a lone punch into his face, killing him on the spot as his gigantic body toppled into B-City and obliterated it.

The drug had left Marugori with monstrous size and power, ranking him among the biggest creatures in the whole series. The Hero Association tagged him a Demon-level danger, a grade that would have risen to Dragon had Saitama not cut him short. His blows loosed shockwaves fierce enough to wreck a city, his stomps could gouge huge craters into the earth, and the pit he left by hammering Saitama spanned better than a hundred meters, while his sheer speed let the towering mutant rattle off rapid combinations despite his bulk.