Vaccine Man

In his default guise Vaccine Man was a towering, bald humanoid tinted purple, his bare body thick with muscle and topped by two rounded antennae. He could also cast off that shape entirely, ballooning into a larger, more elaborate monster armed with jagged teeth, hooked claws, tusks, scales, and spikes, his eyes glowing a solid red. He was even able to revert just a single limb back to normal to conserve energy, a trick he used while reaching for a terrified schoolgirl.

Cast as an anti-villain, he was consumed by a single conviction: that humanity was a plague strangling the natural world, reducing it to machinery, filth, and carcasses. He named himself the wrath of nature made flesh and believed he acted on the direct will of the Earth, so any resistance to his crusade struck him as an outrage. His fury peaked when Saitama treated the confrontation as a casual pastime rather than the cosmic reckoning he imagined it to be.

By his own account Vaccine Man rose in answer to industry's stranglehold on the environment, an apostle conceived by Mother Earth and charged with wiping out humanity so that nature could flourish again. Viewing the planet as one living body in pain, he set about striking the technological hubs where people cluster. He touched down beside A-City, gouging a crater on arrival, then conjured spheres of energy that tore through the skyline. Lightning Max and Smile Man were sent to stop him and were swatted aside, part of a rampage that reportedly blew past thirty-one heroes, some of them A-Class fighters who simply fled. When he turned to crush a sobbing schoolgirl, Saitama intercepted him. Offended that this plain-looking man fought merely for a hobby, Vaccine Man launched into a tirade about his sacred purpose and shifted into his second form, only to be undone by a lone punch. In a running gag, formal credit for the kill later went to King, who happened to stumble over the corpse. Commentators have even mused that, since Saitama's ceiling is unmeasurable, Vaccine Man cannot be ruled out as secretly the strongest monster in the whole series.