Genos vs. Mosquito Girl

Genos vs Mosquito Girl was the battle between Genos and the demon-level monster Mosquito Girl.

Prelude
A mysterious mosquito swarm attacks Z-City, prompting an evacuation. A man takes advantage of the situation to loot stores. The mosquito swarm surrounds him and sucks the blood out of his body.

Battle
After following the swarm to Mosquito Girl, Genos confronts her and fires a small fire blast, which Mosquito Girl easily deflects. Genos begins analyzing Mosquito Girl's behavior and her powerful control over the swarm of mosquitoes. Looking down condescendingly at Genos, she commands her mosquitoes to suck Genos' blood dry. The swarm swirls around Genos, but Genos effortlessly incinerates the swarm, marginally surprising her. Genos bluntly states he will eliminate her and Mosquito Girl laughs at the notion, and their battle begins. Genos runs up a building, jumping up and shortly fights her close quarters. He propels himself back, quickly jumping up and swinging his right fist, as Mosquito Girl meets his attack with her own fist. She then slams him to the ground, but Genos lands unharmed. Deciding to change his form of attack, he launches several quick incineration blasts at her. She swiftly avoids his attacks, and moves closer and closer to Genos until she tears his left arm off with her horn. Mocking the loss of his left arm, she prepares to go after his left leg, only to realize that Genos had ripped off her legs without her even realizing it. She looks at Genos with gradual panic on her face, then attempts to fly away from the battle. Genos blasts her with another fire blast, but the mosquitoes form a barrier and spare her from the attack. While flying in the sky, she contemplates what she's going to do against Genos, then realizes she has plenty of blood to draw from the surrounding animals, and her mosquito minions swiftly drain all the blood from the animals and begins to pour it into Mosquito Girl. Back on the ground, Genos realizes the mosquitoes re-swarming around Mosquito Girl. Coming to the possibility that the blood may be more than a food source for her, he prepares his incineration cannon to end her, but is interrupted by an unknown civilian who is trying to kill a mosquito with bug spray. Saitama soon notices the giant swarm of mosquitoes, and looks at the swarm in panic. Thinking of him as an idiot, Genos tells the stranger to evacuate and the dangers of the conscious mosquito swarm, but the malevolent laugh of Mosquito Girl cuts him off. The horde of mosquitoes crashes down onto the city, destroying windows and buildings as the mosquitoes spread. Taking the advantage of having all the mosquitoes gathered in one place and that there were no civilians for 100 miles, he launches his incineration cannon, effectively eliminating the mosquitoes. Genos calmly states the scenario to himself and how he confirmed there were no humans, before remembering that there was still a man next to him. Suddenly, Saitama, completely in the nude, compliments Genos for killing all the mosquitoes and making them bug off. However, to Genos' utter shock, Mosquito Girl is flying in the sky, completely unharmed and sporting pink colored skin. She says that she no longer needs her mosquito helpers, then slashes a building with just the shock-wave of her attack. Resuming the battle, she speeds towards Genos and slashes his waist. Genos is unable to keep up with her speed, and is thrown up in the air. Bruised, beaten, and outmatched, he realizes the blood also increased her power exponentially. Regretting that he let his guard down, he knows he has no chance of winning, and that he could only self-destruct. He apologizes to Dr. Kuseno as he lives his "final moments", but before he can do so Saitama effortlessly slaps Mosquito Girl and sends her flying, presumably killing her. In complete awe that Saitama was able to finish her off in one hit, he asks Saitama for his name, and much to Saitama's surprise, begs him to take him on as his disciple.

Aftermath
This battle had a significant emotional effect on Genos because he knew his arrogance and the fact that he underestimated his opponent lost him the battle, and had Saitama not been there, he would have surely been killed. In order to become stronger and not make the mistakes he did in this battle, he goes to Saitama's house and begs him to let him be his disciple. Saitama agrees, and the two begin their silent but evident friendship.