Genos vs. Mosquito Girl

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

Prelude
A seemingly conscious mosquito swarm attacks Z-City, prompting an evacuation. A man takes advantage of the situation to loot stores, but 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 control over the mosquito swarm. 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 and engages Mosquito Girl in close quarters combat. 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 onto the ground, but Genos lands unharmed. Deciding to change his form of attack, he launches several quick incineration blasts at her. Mosquito Girl flies towards Genos, dodging his blasts, and tears his left arm off. She mocks Genos, only to realize that her legs have been ripped off. 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 save Mosquito Girl from the attack. While flying in the sky, Mosquito Girl contemplates what she's going to do against Genos. She then realizes has plenty of blood to draw from the surrounding animals, and her mosquito minions swiftly drain all the blood from the animals and begin to pour it into Mosquito Girl.

Back on the ground, Genos observes as the mosquitoes swarm around Mosquito Girl. Coming to the conclusion that the blood may be more than a food source for her, he prepares his incineration cannon to end her, but is interrupted by Saitama, who is chasing 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 Saitama to evacuate, but Mosquito Girl cuts him off with a laugh. The horde of mosquitoes crashes down onto the city, destroying windows and buildings as the mosquitoes spread. Genos uses his incineration cannon to destroy the swarm. Genos calmly states the scenario to himself and how he confirmed that 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' shock, Mosquito Girl is flying in the sky, completely unharmed and with a transformed body. She says that she no longer needs her mosquito helpers, then slashes a building with a single attack. She attacks Genos, slicing halfway through 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 and knowing that he had lost, he prepares to self destruct. He apologizes to Dr. Kuseno as he lives his "final moments". Before he can self destruct however, Saitama effortlessly slaps Mosquito Girl, killing her immediately, and leaving Genos in shock.

Aftermath
The battle gave Genos the realization that his arrogance had led him to underestimate his opponent and lose. In order to become stronger, he decides to visit Saitama's apartment and requests to become his disciple, to which Saitama agrees.