There is but one constant in the One-Punch Man universe: Saitama is the strongest. His strength is limitless, his power absolute, and his opponents are mere fodder for his singular namesake blow. On his quest to cure himself of boredom, Saitama has faced off against god-like beings and has raised his fist in victory on every occasion -- except for one. When Saitama squares off against a mosquito, the lowly insect flies away unscathed, humiliating the titular hero in the process. Is this particular blood-sucking bug the most powerful being to have ever existed?
In One-Punch Man Season 1, Episode 2, “The Lone Cyborg,” a news report heralds the coming of a great mosquito swarm. Saitama, carrying on with his daily routine, steps out on to his balcony to water his plant when a single mosquito begins to pester him. Traditionally when a mosquito lands on one's body, the natural instinct is to swat it with an open palm. Saitama does just that only to gasp in horror as the mosquito flies away from under his palm.
What follows is the most dangerous game between man and bug to have ever been broadcast. Saitama, clearly beyond annoyed, sets his elephant-shaped water canister down and puts on his serious face, one usually reserved for Earth-destroying monstrosities. Saitama's movements cannot be seen by the naked eye as he chases the insect around the small space of his patio, his hands slapping together with such ferocity that his windows rattle.
All of this to no avail as the mosquito simply flutters away. As Genos is introduced, the villain known as Mosquito Girl begins to wreak havoc along with her swarm of mosquitos. Saitama is next seen chasing his foe with a can of bug spray -- the last resort for a man who has accepted defeat.
Given that in the One-Punch Man universe, all manner of superpowered beings exists, there is room for a mosquito-sized hero or villain as well. Unlike the swarm, the mosquito that pestered Saitama acted on its own accord signifying that it wasn't a part of Mosquito Girl's crew. Her mosquitos were easily dispatched by Genos' fire attack. Could this rogue mosquito be an unidentified super-powered being who literally flies under the radar and perhaps only engages heroes in combat to cure itself of boredom?
It wouldn't be that strange for there to exist such a creature given that Saitama's original motivation to become a hero for fun came from a crab-like monster. After becoming the "Caped Baldy," Saitama has gone toe-to-toe with mole-people and sea-creatures as well. His learning of an unbeatable foe the size of an eraser head shouldn't have come as a surprise to him, but it probably shouldn't have annoyed him as much, either.
After all, Saitama is in search of a challenge, one that will give him a rush of excitement the likes of which he hasn't experienced since he took up the daily 100 push-ups, 100 sit-ups, and 10k run challenge. It just so happens that the mosquito -- whatever manner of being it actually was -- provided him with the opportunity. Unfortunately Saitama, like the rest of humanity, shares the opinion that the only good mosquito is a dead mosquito.
Since their fateful encounter, the mosquito has never returned, meaning that the true nature of its powers may be forever hidden. It may have been the most powerful being in the One-Punch Man universe... or simply one of many gags with no deeper meaning than that, even for heroes, mosquitoes are super annoying.