One Punch Boss

Prologue

In a world where heroes and monsters battle one another for dominance every other day, not a lot could surprise people. Which is why it was doubly surprising when City Z suddenly cut itself off from the other cities. The Hero Association was especially alarmed, since every hero in the city was found unconscious outside of the Association HQ, with the exception of Bang, who refused to tell anyone what was going on. Every hero they attempted to send in was found in the same state. They considered sending in an S class, but opted to ignore it since, from what they could see, City Z was still intact and its citizens still visited the other cities. But then there was the new visitation policy. You had to apply for a permit and sign no less than three NDAs in order to be allowed in. It was very odd, and unnerving to some. No one could have guessed that such a large change was brought about by a single man. A single, impossibly powerful man.

A humanoid purple creature of unreasonable size stomps through City A, destroying everything in its path and then some. In City Z, in a modest mansion (what an oxymoron) a bald man in a simple black suit with coattails and a white button up watches the news as they broadcast the damage done by the monster. "Guess I'll go." The man puts on a pair of simple black gloves that match the suit before he leaves his home. Throughout City Z, many civilians, both human and monster watch the same news broadcast, wondering if this powerful monster will be their new neighbor.

The bald guy arrives in City A in time to see the purple monster reach out to crush a little girl. That won't do. He dashed through the monster's clenching fist to grab the girl and place her down somewhere safe with a victorious smirk. The monster stares down at the bald guy with interest. "You're fast. Who are you?"
The bald guy jabs a finger at his chest, his facial features sharpened with seriousness. "Names Saitama, and I run one of the nearby cities. And you are?"
"I am Vaccine Man. I was born from the pollution that mankind has created. The earth is a living organism, and humans are nothing but a disease, killing her!" He grows larger and more monstrous looking as he monologues. "In order to wipe out the humans and save the earth, she gave birth to me!" Vaccine Man is truly monstrous now. "And I shall cure Mother Earth of humanit-" he is quickly silenced by a backhand from Saitama, knocking the purple monster on his ass.
"How boring."

Shrunk back down to a more humane size, Vaccine Man stares at Saitama in shock. He didn't even appear to TRY and attack him, just a backhand sent him sprawling. A LAZY backhand at that. "You are no normal human."
Crossing his arms, Saitama smirks. "Nope. Listen, when a monster gets this kind of attention from the news, I give them two options. You could come to City Z to work for me. With your talents, I'm sure I could find something for you to do. Or, I could just kill you right now since I can't risk letting you trash my town. What's it gonna be Vaccine Man?"
Vaccine Man didn't have to think too hard about it, considering Saitama's earlier show of raw strength. "Fine."
"Sweet." In the blink of an eye, Vaccine Man is over Saiatam's shoulder. "Let's get out of here before heroes show up."

Vaccine Man was put to work in City Z by using his ability to change size in order to make construction cheaper and more efficient, the lack of heavy weight vehicles that are normally needed to move the materials around reducing the carbon emissions, and he also had a side job consuming the air pollution from the power plant. Overall, not a bad gig for an environmentalist monster.