4 years before the start of the series, Genos was a regular boy who lived in a small town. Then one day, out of nowhere, a random, psychotic cyborg attacked Genos's hometown- completely destroying it and killing Genos's family in the process. Afterwards, a scientist for justice named Dr. Kuseno passed by through the town looking for the cyborg. Genos then begged Dr. Kuseno to transform him into a cyborg in order to help track down the one that murdered his family and destroyed his town. The doctor agreed, and ever since then Genos has been hunting down evil organization and fighting for justice.
House Of Evolution Arc
Genos is first seen in an open field, investigating several animals that were killed and drained of blood before heading off to Z-City to face off against the culprit, Mosquito Girl, who had just finished draining a robber's blood with her mosquitoes during a city wide evacuation. He is seen putting up a fairly even fight with the villain with him tearing off her legs after she tore off his arm. Genos is suddenly distracted by the appearance of Saitama chasing down a mosquito and immediately tells Saitama to leave if he did not want to get hurt only to realize too late that Mosquito Girl was flooding the immediate location with mosquitoes. Genos counters with an attack that wipes out the mosquitoes throughout multiple city blocks and ends up being worried that Saitama might have been caught in the attack only to see a naked Saitama congratulate him from behind.
Mosquito Girl suddenly appears in her strongest form after all the mosquitoes gave her all the blood that they've absorbed and easily overpowers Genos. Genos, in an attempt to defeat the villain, tries to blow himself only to be shocked when Saitama easily kills the villain by slapping her towards a nearby building. Before Saitama leaves, Genos asks for his name and asks how he could contact Saitama in order to become his disciple.
A few days later, Genos visits Saitama in order to tell him his backstory and explains why he wanted to train under Saitama which an initially hesitant Saitama agrees to. However, they are suddenly attacked by members of the House Of Evolution. After Saitama easily deals with the initial attackers, Genos fights against another member who attempted to attack Saitama, Armored Gorilla. After defeating Armored Gorilla off panel, Genos interrogates the gorilla about the whereabouts of the House Of Evolution. Armored Gorilla initially avoids answering Genos' questions but complies after seeing that Saitama defeated his superior.
Later on, Genos and Saitama successfully infiltrated the House Of Evolution's headquarters but the cyborg is easily defeated by the House Of Evolution's strongest creation, Asura Rhino. After waking up, Genos attempts to defeat the Asura Rhino but is easily defeated again. When Saitama reveals that his source of power was simply an intense training regiment to Genos, Asura Rhino and the House Of Evolution director, Dr. Genus, The cyborg voiced his initial disbelief at Saitama's backstory only to be silenced by Saitama's statement that it was all he actually did to get stronger. Asura Rhino transforms into his strongest form in anger and initially appears to overpower Saitama in Genos' perspective only to be shocked when Saitama easily kills Asura Rhino with a single hit after realizing that there was a sale in the market that day. Genos reassures his master that they could still make it to the sale if they hurried.
Paradise Group Arc
Genos is with Saitama in the latter's apartment, with Saitama complaining of his lack of acknowledgement from the public after his encounter with Speed-o'-Sound-Sonic, who admitted to never hearing of him. Genos then informs Saitama that he could attain fame if he was registered as a licensed Hero and the two decide to register themselves.
National Hero Registry Arc
Genos registers for the Hero license exams and aces the physical and written exams, making him an official S-Class hero, the highest ranked heroes in the Hero Association. He is then shocked by the fact that Saitama is only a C-Class hero, the lowest ranked heroes in the organization, because he barely passed the written exams even if he aced the physical exams.
Some time afterwards, Genos spars with Saitama, asking him not to hold back during the sparring session to see the source and limits of his power but is easily overwhelmed by Saitama's own power. They later eat at restaurant that evening when Sweet Mask, a fellow licensed hero and celebrity, approaches Genos to welcome him in the organization. Genos is intimidated by the hero's cold demeanor and makes him leave. Walking home, Saitama and Genos talk about Saitama's perspective of being a hero, which inspires Genos to move in with Saitama. Saitama, though initially hesitant to let Genos live with him, accepts his request when Genos offers to pay rent.
Genos is a youthful looking young man with an entirely mechanical body. He as spiky ash-blonde hair and purely black eyes with yellow irises. He also has two silver piercings and normally wears tank tops and jeans with black or brown boots. He has received several different mechanical upgrades throughout the series, each one changing his appearance drastically.
Genos is a very determined man with a strong sense of justice who is constantly striving to become stronger. Because of his resolve and history, he will do whatever it takes to become more powerful. This leads him to greatly respect Saitama and frequently ask him to train him.
Genos usually serves as comic relief to Saitama's actions, especially during battles or simply when referring to his baldness. He stays with Saitama out of belief he will become stronger, and greatly respects him. However, Genos is a serious character and there are times when if he feels annoyed or bewildered he will lash out, such as when Saitama explained the source of his power. Moments like this highlight Genos's level headed qualities, and the fact that he is willing to do anything to become stronger, even if it is to turn against his master.
Genos also has the tendency to either underestimate his opponents in battle, or let his guard down. This was shown his battle with Deep Sea King and Carnage Kabuto. He also has a sense of respect for himself, and feels disappointed whenever he fails to reach a goal, such as in his battle with Mosquito Girl or his attempt to stop the meteor from falling on Z-City.
Powers and Abilities
Genos is incredibly powerful, being not only an S-Class hero, but a genius fighter with large battle experience and several techniques. Despite these things however, a running gag in One Punch Man is Genos's struggle to battle powerful characters, only for all of them to get defeated by Saitama in one punch.
Genos has an entirely mechanical body that's been equipped with combat-ready cybernetic modifications. These include several weapons and canons, as well as sensors, scannors, and boosters. These cyborg enhancements enable Genos with vast amounts of super strength, speed, durability, and stamina.
Genos has immense physical strength, and is capable of obliterating the face of a cliff with a flurry of punches. He was seen cracking bedrock with a simple drop kick, and was able to trade blows with someone as powerful as the Deep Sea King, who earlier blew a hole through a nuke shelter with ease. It can be assumed that while not nearly as strong as Saitama, Genos's strength level allows him to lift/press at least 70 tons.
Genos is extremely fast, being able to keep up with a casual Saitama (albeit for a short while) the latter who is in the same speed tier as Boros. He was also keeping up with Speed O' Sonic, who is massively hypersonic and among the fastest characters in the series. This would indicate that Genos is capable of moving at a minimum of MACH 2 (1440 mph), without boosting his baseline physical output.
Superhuman stamina and durability
Was able to keep fighting perfectly well despite losing an arm, fought after having his neck broken by Carnage Kabuto, and tanked the air pressure from a "death" punch from Saitama (the same air pressure which annihilated the landscape behind him)
Genos cybernetic endo-systems provide him superhuman sensory capabilities. he can also form complex offensive weapons that are capable of significant destructive power. A single energy blast from his hand (one of his baseline weapons) can obliterate a seven story building. These onboard systems arrays can be upgraded by Dr. Kuseno to provide Genos with additional capabilities.