At age 15, Genos was still a human being living a happy and peaceful life along with his family. One day, a crazy cyborg attacked their town and destroyed it, killing his family. Genos miraculously survived, but he wouldn't survive long alone in his ruined town if he wasn't found by Dr. Kuseno who passed the town pursuing the cyborg. Genos begged Dr. Kuseno to transform him into a cyborg, who complied. Genos was reborn as a cyborg fighting for justice and promised Dr. Kuseno that one day he would find and destroy the cyborg.
For the next four years, Genos traveled from town to town defeating monsters and evil organizations, but was unable to find the whereabouts of the cyborg.
Saitama Introduction Arc
House of Evolution Arc
While Mosquito Girl is in the middle of her rampage in Z-City, she notices that quite a few of her mosquitoes have died and looking down, she spots a form swarmed by mosquitoes. She sends more bugs at the form, but they are all blasted away, revealing Genos. He has a stern expression on his face and declares that he will eliminate Mosquito Girl. Genos then fires a blast at her, but she easily dodges his attack and dives at him, taking his arm in the process, but in return he takes out both her legs. Mosquito Girl then flies away and summons all of her mosquitoes to give her their blood. Genos says that her efforts are useless, but is concerned when he sees the amount of bugs gathering around her.
As he is about to charge his laser, Saitama comes running out on the street. Genos warns the so-called civilian, but Mosquito Girl's swarm of mosquitoes comes crashing down on them in an instant. Genos releases his heat beam and burns all the bugs, while telling his foe that making them attack in one big chunk makes it far easier to destroy them. He realizes that Saitama was still in the area, but is shocked to see that he is fine, with only his clothes burned.
Mosquito Girl, now evolved, rushes down and appears behind Genos, cutting him almost completely in half. Genos then spins around to attack her but she dodges and begins to attack him continuously to the point that he attempts to self destruct in order to kill her. Before that can happen, Saitama ungracefully smacks Mosquito Girl in the face, sending her crashing through a building and into the distance, killing her. Genos, greatly surprised by this, asks Saitama his name and for Genos to become Saitama's disciple.
A week later, Genos shows up at Saitama's apartment, having been fully rebuilt, to discuss being Saitama's student. Genos rather long windily begins to tell Saitama about his past, but Saitama tells him to come back when he can sum his story up with a max of ten words. Sometime after, Genos simply asks Saitama to teach him to become strong like him. Saitama warns Genos that it's going to be tough and agrees to train him. But at that moment, Genos turns, saying that he senses a presence moving at high speeds towards them, when Kamakyuri smashes through the roof, only to be swiftly killed by Saitama. Genos and Saitama leave the building to find Frog Man and Slugerous on guard. Genos tells his master that he will take care of them, but before he can finish his sentence, Saitama has already smashed both creatures head first into the ground. Then, Saitama unexpectedly gets dragged down underground until only his head is showing. Genos starts to rush to Saitama's aid, but Armored Gorilla suddenly bursts through the wall behind them.
Armored Gorilla then attacks Genos who easily stops the attack. He says that he needs to ask the creature some questions. He then notices Beast King approaching Saitama, who is still buried and goes to help him but gets grabbed by Armored Gorilla who he then blasts away, and starts to run to Saitama again. Armored Gorilla attacks again and Genos is forced to jump away. The cyborg reveals that he is affiliated to the House of Evolution. Genos recognizes the name and asks what they want of his teacher. Armored Gorilla doesn't reply and simply states the he must destroy Genos. Saitama is then shown taking out Beast King and Ground Dragon and approaching Genos who is standing on an incapacitated Armored Gorilla, having defeated him in an off-screen fight. After showing Armored Gorilla that Saitama has killed Beast King, he agrees to tell them whatever they wanted to know.
He summarizes that the doctor has taken interest in Saitama's body and Genus clarifies that he wants to use Saitama's god-like physique to build the new human. As they take off to the House of Evolution's base, Genos asks some last questions. He asks about the House's involvement with cyborgs and Armored Gorilla replies that he's the only cyborg of the organization, which disappoints Genos. Four hours later, Genos and Saitama have arrived at the House of Evolution's base. When they find it, Genos promptly destroys the building and part of a mountain in the distance. Saitama is surprised of Genos' quick actions and notices a hatch on the floor and rips it open before Genos and him head down into the lair.
As they're walking down the hallway, Genos notices several life forms, two of which are coming at them at high speed. Carnage Kabuto appears, carrying Dr. Genus, who then points out Saitama, identifying him as Carnage Kabuto's target. Carnage Kabuto then throws Dr. Genus to the side and smashes Genos against the wall. The creature challenges Saitama to a fight, and he agrees on the grounds that Carnage Kabuto made Genos look like "modern art." Carnage Kabuto then shows Saitama to a rather large room and they are about to fight, but Genos suddenly appears and blasts Carnage Kabuto. Genos then rushes at Carnage Kabuto and hits him with a barrage of punches, but it doesn't affect the monster and he smashes Genos in the face, throwing him into the ground and causing his face to crack and his left eye to fall out. Saitama catches him and says he'll handle the fight, but Genos, stubborn as he is, sends a devastating blast at Carnage Kabuto. However, the opponent uses his breath to blow the attack back and blasts Genos instead, causing Genos' hair to puff up into an afro.
Saitama gets serious and proceed to challenge Carnage Kabuto, but the latter stops his assault when he detects Saitama's abnormally high strength and questions Saitama about the source of that strength. Saitama decides to reveal the secret to his power. Genos is amazed and is eager to finally know, but inwardly questions whether it is wise to divulge the secret to his enemies. Saitama explains that it is simply the result of hard work and training and obviously, Genos doesn't buy it and is skeptical. He tries to persuade Saitama to let him know the real secret, but Saitama simply replies that it's just that. Carnage Kabuto goes berserk and pummels Saitama, but he unexpectedly delivers, but one punch to the monster and destroys him. After Carnage Kabuto's death, Genos shows Dr. Genus that Saitama was upset about being unable to make the supermarket special sale on time and tells Saitama that they will be able to make it if they arrive in four hours. After they left, officers of the Hero Association find the results of Genos' blast that destroyed the top eight floors of the House of Evolution and identify Genos as the perpetrator of that blast.
Paradise Group ArcAfter his fight with Carnage Kabuto, Genos is getting repaired by Dr. Kuseno, who suggests that his next round of upgrades, the ones Genos uses in his fight with Saitama, would enable him to defeat Saitama. Genos then visits Saitama, and reveals to Saitama that he is not famous because he never registered himself with the Hero Association. He tells the history of the Association to Saitama and they agree in getting them both registered.
Hero Association Saga
National Superhero Registry Arc
Genos and Saitama sign up at the 6th official hero certification exam venue. After he finishes the physical and written exam, he is greeted by Saitama in the changing rooms. They agree on how both tests were super easy and that they'll probably get full points too. Genos was also interview on whether he was the one who destroyed the House of Evolution's building. Genos confirms this and the officials then make an exception and immediately promoting Genos to S-Class based on prior experience. A hour later, they receive their scores, revealing that Genos received a perfect 100 and is certified as a S-Class hero, though he wonders what that is. Genos becomes enraged when he hears that his teacher scored 71 points ranking him as a C-Class. They are then told to go to a lecture hall for a seminar of the successful candidates.
As they arrive in hall 6, they are congratulated by A-Class hero Sneck. He gives them some advice on being a hero and the rankings, though Genos isn't interested in it. As they leave the seminar, Genos is glad that he finally becomes a real disciple of Saitama and counts on his guidance, though Saitama isn't actually glad with it, and they part ways soon after.
The next day, Genos and Saitama appear around a mountain area, with Genos sporting new black mechanical arms. Genos checks his phone, stating they are at the last place of their respective classes and that they didn't receive hero names yet. He puts the matter aside and thanks Saitama for accepting his "unreasonable request". The two prepare for battle, though Saitama believes it to be a sparring match, however Genos will attack with everything he's got. Genos positions in a battle stance and aims his left arm at Saitama, ports of his arm opens revealing several cannons.
He fires a single massive beam at Saitama. As it reaches Saitama, he dodges it, however Genos immediately retaliates by dashing towards Saitama. He takes a hold of Saitama and crashes him to the mountain, punching several times across the wall of the mountain, but he soon realizes that he was attacking Saitama's after-image. Genos immediately detects that Saitama is running away and intercepts him by landing in front of him and firing a massive beam. Genos believes he hit and defeated Saitama, only to have Saitama poke him in the cheek from behind and proclaiming victory.
Genos becomes angry and attacks him, but Saitama jumps away unscathed. Genos then reminds Saitama the rules of the fight, one of which is to continue fighting until he is unable to. They continue the fight, in hoping that he will discover insights of Saitama's power, however Saitama shows impressive fighting prowess and lunges to punch Genos, but instead flicks him and invites Genos to eat udon. Genos watches Saitama walk away, as he is still flabbergasted of the force of the last punch, that destroyed a mountain behind him, realizing his teacher is on a whole other level.
As Genos and Saitama visit an udon shop, Saitama decides to challenge him to a spicy udon eating contest, which ends with Genos the victor and the two tied in competition results. Sweet Mask then meets with Genos, who assumes that he had come to rookie beat him and tells him to stop wasting his time, but Sweet Mask denies this and simply states that he met with him to welcome him to the Association. Saitama and Genos part at a bridge and walk in opposite directions, initially implying that the two would walk separate paths as heroes. This is later disaffirmed, when Genos simply returned to pack his belongings to move into Saitama's apartment. Saitama does not agree to, though he changes his mind when Genos offers to pay him rent.
Rumored Monster Arc
Genos then proceeds to write in his diary on his teachings. Genos then informs him that C-Class heroes will be excluded from the hero registry as they are not deemed worthy by the registry if they do not work. Saitama shocked at the discovery, decides to do heroic deeds. Genos wants to accompany him, but is convinced by Saitama to aim towards the top ten ranking of the S-Class, as Saitama believes that if Genos were to accompany him, he would inadvertently steal all the credit with his mere presence.
While Saitama was cooking kombu, plucked from Kombu Infinity, Genos wonders why there is a pile of kombu outside, which Saitama tells him it was cheaper. Genos asks Saitama if he thought that kombu would let him regrow his hair, that it will actually not, as it was not proven as a genuine medicine, to which Genos receives an angry response from the infuriated Saitama.
Giant Meteor Arc
Saitama and Genos are relaxing and reading the newspaper. Genos states that Saitama went up in the C-Class ranking from 388 rank to 342 and that while he is still last in the S-Class ranking, in popularity he is ranked 6th, by public votes, shocking Saitama. Then, he mentions several comments about him, but is not impressed since they are all only based on one single picture of him. Even though the world does not acknowledge it, he is still astounded by Saitama, which creeps him out.
Genos receives a call to go to the Hero Association for unknown reasons. When he arrives at Z-City branch, with a suitcase, he is greeted by the third-ranked S-Class hero Bang, who reveals that the Hero Association ordered a mass evacuation and that he and Genos are the only S-Class heroes who showed up to prevent the disaster. He explains that they were given an impossible task, a Dragon Level Disaster is coming to Z-City in the form of a giant meteor. Genos is advised by Bang to evacuate before the meteor impacts. Genos wonders what Bang is going to do, which he replies that he decided to protect his dojo, but as he turns around to show his fighting style, Genos already left.
Genos is later shown to be leaping from rooftops, observing the panic and prepares himself for an attempt to stop the meteor. Genos activated his "Arms Mode" suitcase and throws it in the air, forming it into a pair of arms and attaches to himself, surging electricity through his body. As he decides to destroy meteor in order to protect his teacher, a large, humanoid, unmanned drone flies above him towards the point of arrival of the meteor.
Genos reveals the large robot to be another S-Class hero called Bofoi, and asks for his help in stopping the meteor. Bofoi refuses and states that he is only there to test a new weapons system. He also advises Genos to call fellow heroes by their nicknames in professional situations, and stating that his is Metal Knight. The meteor looms closer so Genos runs off to a different roof and prepares himself to attack the meteor. Before he can do anything though, Bofoi lets off a massive volley of missiles, which in the end do nothing to stop or slow down the meteor.
Genos starts to prepare, but starts to doubt his abilities when Bang shows up and calms him down. Genos thinks of Saitama and comes back to his senses. He opens up his chest to reveal his Power Core which he attaches directly into the shoulder of his left arm. He tells Bang to get down and then launches a massive attack at the meteor. His attack does nothing to the meteor unfortunately and Genos runs out of power and collapses on the roof. Saitama shows up and destroys the meteor, causing pieces of it to rain on the city, while Bang carries Genos to safety.
Genos makes a brief appearance at the end of the arc in his and Saitama's apartment, regretting his inability to stop the meteor. After a short discussion he reveals that his rank has been increased from 17 to 16, Bofoi's from 7 to 6 and that Saitama has been raised from rank 342 of the C-Class to rank 5 of the C-Class. He also explained the Disaster Level System. When Saitama decided to go outside to check the damage, Genos stayed behind. Genos is seen looking up comments on the website about Saitama destroying the meteor. Then later stopping Saitama from getting all worked up from someone calling him bald by saying they should go home. He later tells Saitama that even though the whole world would go against him, he would continue to follow him.
Sea Monster Arc
Genos is in their apartment discussing Saitama's ranking after he killed a Seafolk from the other day. When he was about to explain the top B-Class hero, he gets a call that J-City is under attack and there was no competent heroes nearby to help. He and Saitama head out despite it being far away from where they are residing. Genos and Saitama ran towards J City in the pursuit of the Sea Folk, but decided to go ahead of Saitama. On his way, he met a naked Speed-o'-Sound Sonic who had just escaped from the Deep Sea King and was told he stood no chance against the monster, before he disappeared. He was the first to arrive in the Evacuation Shelter where the Deep Sea King just took down four heroes. Genos uses Incinerate on the Deep Sea King, punching him out of the shelter with the crowd later cheering for him. Genos lets his guard down, and gets one of his arms ripped off. He tells the people to run while he distracts the monster, since victory is no longer assured. 
He put up a fight and traded blows until he was hit by the Deep Sea King's Acid Spit to save a girl, melting his body, which ultimately disabled him. Genos was then punched by the Deep Sea King out of the shelter. Right before he was about to be finished, Mumen Rider came and engages the Deep Sea King. Genos begs Mumen Rider to leave and struggles to get up to save him.
However, Saitama arrives and easily defeats the Deep Sea King with a single punch.  In the aftermath, the people were thankful but one criticized heroes for being defeated by the monster, despite their efforts. Genos angrily tries his best to get up to intervene, but is too weak to get up. Annoyed by this, Saitama pretended to take all the credit by himself, with the purpose of giving credit to those who gave their all. Realizing this, Genos' respect towards his teacher grew more.
After the incident, Genos and Saitama get a delivery of fan mail from the Hero Association. Genos is praised, while Saitama gets a hateful letter, calling him a cheat. Genos angered by the letter stated he will find the senders.
Alien Conquerors Arc
Genos and Saitama are at Bang's dojo, where they are demonstrated the Water Stream Rock Smashing Fist and are asked to join, to which Genos refuses. Bang top disciple Charanko was angered by their statement and challenges Genos, who Genos quickly defeats. Suddenly a Hero Association staff member arrives, informing Bang and the others that an emergency summons has been issued to S-Class heroes. Genos arrives to the Hero Association with Bang and Saitama; they are greeted by Atomic Samurai.
He later goes to the roof of the Hero Association Headquarter with Child Emperor, Superalloy Darkshine, Tatsumaki, and King. There he asks Tatsumaki if he can help, only to be declined. Afterwards, Genos appears beside Child Emperor and Drive Knight and is told by Drive Knight not to trust Metal Knight. Later when Metal Knight's robot appears, Genos asks what Metal Knight's intentions are. Genos is relieved when he sees that Saitama is okay after the crash. However, soon Tatsumaki erupts into a fit of anger for being ignored and begins insulting Saitama. Genos retaliates by calling her a shitty brat. Unfortunately, Tatsumaki did not take that comment nicely and launches Genos at a large chunk of debris.
King ArcWhile shopping with Saitama in M-City, Genos recognized King on the streets, when suddenly a robot monster by the name of G4 appears to fight King. Genos proposed to watch the fight, to see King's abilities. After King fled the scene, Genos begins to fighting G4. Because the fight between the two will take a little longer, Genos advises Saitama to go without him. During their fight, Genos melted the robot, only to reveal a smaller pilot inside. He proceeded to fight the robot, catching him in a trap and defeating G4.
Human Monster Saga
Garou Introduction Arc
Genos then went to Dr. Kuseno with the remains of G4 to get repairs. Genos tells Dr. Kuseno that he wants to become stronger. Dr. Kuseno replies that Genos' master has given Genos a very reckless goal and asks whether Genos can bring Saitama next time Genos comes. Their conversation continues with the discussion of the cyborg that destroyed Genos' hometown. Dr. Kuseno ask Genos whether he found any clues about the cyborg's whereabouts, which Genos replies with no. However, Genos reaffirms that he will destroy the cyborg, once he finds him. Dr. Kuseno reminds Genos that he is not alone and that the conflict with Mad Cyborg is their conflict and not just Genos'.
The Blizzard Group Arc
While at home, Genos inform Saitama about their promotions in their respective ranks, knowing Saitama doesn't report his work, Genos believes his master is finally being recognized. Genos suddenly sensed something coming towards their place and went to check it out which is revealed to be Speed-o'-Sound Sonic who claims himself as Saitama's rival. Not wanting to disturb a busy Saitama, Genos decided to take him down on his own.
They are on equal footing, with Sonic too fast for him but he, too durable from his attacks. As he tried to land a blow against Sonic, he accidentally caught Fubuki who is fighting Saitama in the way with it's impact. Saitama asked him what's going on and he replies with an apology for messing up the place. He suddenly recognized Fubuki and misinterpreted her position as a failed attempt of taking down Saitama. Fubuki was surprised that he, an S-Class Hero actually called himself a disciple of a B-Class Saitama. Sonic attacks him again, this time with exploding shurikens but was unharmed.
Despite his loss in speed after this, he is durable enough to take all of Sonic's attacks and decided to increase his speed to catch up. Sonic barely evades this attack, but Genos manages to cut off his hair. This angers Sonic severely and he stops taking the fight lightly.
Sonic uses Four Shadows Burial, thus preventing Genos from tracking Sonic. As a last resort Genos decides to try to blow up the general area. However, Saitama knocks down Genos before the cyborg can do so. Saitama then brings the fight into his own hands. While watching Sonic fight Saitama, Genos admits, Sonic must have worked hard to gain his abilities. Afterwards, Genos, Fubuki, and Saitama go to Saitama's apartment and listen to Fubuki discuss her ideology.
Hero Hunt Arc
Genos is seen talking to Charanko at Saitama's apartment. He reveals that Garou has been put on the Hero Association's wanted list of vicious monsters and that Bang has volunteered to get rid of Garou. When Saitama comes back with a wig, Genos comes to believe that Saitama is very desperate and calls Dr. Kuseno to make an appointment to give Saitama a hair transplant. Saitama then explains that it is simply a disguise for the tournament and invites Genos to come to the tournament with him at C-City.
Monster Raid Arc
Upon watching the victory of Saitama against Zakos, Genos commented that the tournament will end, before Saitama could learn any martial arts. After witnessing Suiryu's victory against A-Class hero [[[Lightning Max]], he wondered the number of people with power that exceeds that of a heroes and wonders if Suiryu's abilities will satisfy Saitama's desire to fight. Genos receives information about the attacks of an unknown group of monsters in the cities and targeting heroes. In addition there are monster in C-City, thus Genos has to do his hero work to eliminate the monsters while Saitama focuses on the tournament. As Genos eliminates monster nearby the stadium, he receives a call from the Hero Association about the remaining monsters in C-City. He easily defeated many monster and eventually encounters Face Ripper before the monster is able to kill a civilian. Face Ripper manages to chip away parts of Genos' arm, but Genos ultimately catches the monster off guard and is able to land a fatal blow. Genos is called again by the Association about the location of Awakened Cockroach.
During the fight with Awakened Cockroach, the monster was able to block and dodge every attack by Genos, while sending out his own attacks. During their fight, Awakened Cockroach has the upper hand, until Genos decides to use instant adhesive to trap the monster, just in case if Genos encounters a speedster villains like that monster, including Sonic. However before Genos could kill the monster, Awakened Cockroach sacrifices his feet to get away. Furthermore, before Genos could make up his mind about returning to the stadium, another being rushes to Genos. Genos was later seen defeated by Gouketsu, while on his way to the C-City Super Stadium.Genos wakes up after being defeated by Gouketsu. Genos decides to call for Dr. Kuseno's body recovery drones, while noting that Gouketsu is on a whole different level.
The following section contains Webcomic spoilers. You have been warned, manga-only readers.
Monster Association Arc
Genos appears to save Death Gatling from Garou and attempts to fight him but is overwhelmed and almost killed before Bang and Bomb show up. Genos fights Garou and is nearly killed by him before Zombieman gets Garou's attention.
Fall of the Hero Association Arc
Genos appears to save 4 heroes from the Forest King and incinerate him. After finishing his job, he comes back to Saitama's apartment and complains about the small space of the new apartment. He is going to request the Hero Association for bigger space but Saitama stops him since he likes small space since he likes claim environment.
After discussing about the living area, he requests Saitama to duel him the second times while Saitama begins to caution about the rising frequency of monsters these recent days. During the fight, he used his energy core to create bright light to blind Saitama and prepare to punch him. However, he stops in middle of his fist from a distance of Saitama's face and asks him whether he is getting stronger or not. At first, Saitama thinks that Genos changes his parts frequently and doesn't want to let him down so he states that Genos has gotten stronger. Despite that, Genos feels that Saitama doesn't concern about his progress but he puts it aside as his own thinking.
At this time, he begins to think to of changing the environment to get stronger since his goal to reach the top of the Hero Association doesn't put him on par with Saitama. Therefore, he begins to ask Saitama whether he knows about the new Hero Association called Neo Heroes or not.
Appearances in Other Media
Genos is tasked by a member of the Hero Association to destroy a cat-like mister a Grizzly Nyah. 
After being called over to Bang's dojo, Saitama learns that Bang got a whole bunch of lobsters from his brother and needs someone to help him eat them. Genos complains about them being called out for something insignificant, and tells Bang to call someone else if he needs help eating leftovers, at which point Saitama tells him to stop, saying that they're only helping a lonely old man with his food problem. After Charanko arrives with the vegetables, they start preparing the hotpot while talking about a recent dojo hunter that has been attacking other dojo's. Saitama notices that there isn't any cabbage, interrupting the conversation about the dojo hunter, which annoys Charanko. After seeing Saitama look bummed about the fact that there isn't any cabbage, Genos angrily asks Charanko why he planned to make hotpot without cabbage. Saitama tries to say that it's fine, but Genos keeps arguing with Charanko because he does not feel like he even feels sorry for not having bought cabbage. After Charanko challenges Genos to a match, Bang tries to calm down the situation by saying that he will go buy the cabbage, and asks Charanko to get along with the other two.