In the past, Iwao Oguro was a Pro Hero known as O'Clock, while in China he was known as Ciupan Seikyun (Hyper Quadfist). He was a hero specialized in fighting crime, sometimes solo and some time he'd work with the Police Force, being responsible for taking down an impressive number of illegal organizations. His career as a Pro Hero came to end when, when he stormed a building, he met All For One, who stole his Quirk Overclock, leaving his face split open.
Iwao survived the encounter, though many left him for dead. The loss of his Quirk and the end of his career caused Iwao Oguro to fall into severe depression, beginning to drown his sorrows with alcohol.
Iwao has a daughter, Tamao Oguro, with whom whose relationship became strained after she expressed her desire to become a musician with her friends, only for him to yell at her in protest and told her to stop hanging out with that group of people. These discussions caused Tamao to leave home at some point. 
Following Tamao's sudden departure, Iwao Oguro's wife is hospitalized and is currently unable to speak properly. In an effort to track down his missing daughter, the former Pro Hero becomes a vigilante. During his first investigations, he discovers that the body of his daughter has been controlled by a parasitic bee known as Kuin Hachisuka, who works for a mysterious criminal organization (later known as the Villain Factory) that traffic with Trigger and that seems to be responsible for the vegetative state of his wife.
Since then, Knuckleduster is looking for a way to rescue his daughter Tamao and put an end to Villain Factory's operations. To carry out his objective, he uses several aliases to hide his identity, as well as to have contacts with both the criminal world and the forces of law to obtain information and supplies.
During his investigations, Iwao met Koichi Haimawari for the first time. Drunk, he was fighting with a thug he considered suspicious to get information about the Trigger when Koichi intervenes to stop them. The thug leaves angrily after hitting Koichi, for not considering that he was the victim of an unprovoked assault by a drunkard. Despite the beating and bleeding from the nose, Koichi offers Iwao some tissues for his injuries. Iwao decides to leave without saying anything to him, barely showing his face, and smiling at the boy's good heart. Koichi's selfless and altruistic behavior prompted Iwao to rethink his current lifestyle and research methods.
Knuckleduster meets again Koichi Haimawari, along with Kazuho Haneyama, while they are assaulted by Soga Kugizaki and his mates Rapt Tokage and Moyuru Tochi, and decides to intervene, defeating them with ease. After that, he examines their tongues to see if they’re discolored, but cannot find anything. However, impressed by Koichi's bravery, Knuckleduster decides to take him as his disciple to make him a hero, even though Koichi doesn't want to be his disciple.
Next morning, despite Koichi's refusal of his offer, Knuckleduster goes to his penthouse. He explains to him and Kazuho the whole issue referred to Trigger, a drug that gives a boost to an individual's Quirk while also weakening their sense of reason, turning them into Instant Villains. He tells them that is characterized by leaving on its consumers’ tongues a characteristic black discoloration, which is what he was checking for on Soga and his friends. Knuckleduster explains that they will beat them, make them talk about the dealer who gave them the drug and cut off the circulation of Trigger. Koichi thinks that Knuckleduster is not being serious.
However, to Koichi's misfortune, Knuckleduster is dead serious, and his method of investigation consists of punch first and ask question later. Pop☆Step arrives and decides to help them by searching on the internet for "sudden villain appearances" which will allow them to pinpoint the location of the Trigger Junkies. In that moment Knuckleduster spots Mario Kugutsu, a salesman who looks suspicious to him, and tries to interrogate him by force.
When he's about to use violence to force Mario to speak, at that moment Shota Aizawa appears and stops Knuckleduster under the suspicion that he is a villain, which causes Knuckleduster and Eraser Head to start fighting, much to Koichi and Pop☆Step's shock. That moment is used by Mario to escape, and Koichi and Pop chase him after realizing that Knuckleduster was right in his suspicions.
It turns out that Mario Kugutsu is a Trigger dealer for the Villain Factory, who has recently supplied samples of Trigger to Soga and his friends, who appear to be taking revenge on Koichi and Knuckleduster for the beating they suffered in their last quarrel. Mario, Soga and the others inject the drug into themselves, causing them to become Instant Villains, and attack Koichi and Pop.
Meanwhile, Shota decides to stop his fight with Knuckleduster as he has realized that Knuckleduster is not a villain after all, just a stubborn Quirkless old man. At that moment, they hear screams caused by the villains' persecution of Koichi and Pop, and they collaborate together to defeat and arrest them. During the battle, Knuckleduster witnesses Koichi's heroic spirit in saving Pop from Mario.
After the battle, Shota thanks him for his assistance but finds his actions to be dangerous. Knuckleduster does not mind helping since he is natural to cooperate with Heroes. He says to Shota that Heroes need people like him since he is free to deliver justice without restrictions. Shota considers his reasoning ridiculous and finds him to be a troublesome old man, although Knuckleduster feels praised for his words and leaves.
Then, Knuckleduster reunites Koichi and Kazuho, and praises the young man for his heroism, telling him that despite his refusals of his offer to make him a hero, he still aspires to be one. Knuckleduster encourages Koichi to chase after his aspiration to be a Hero. Koichi finally accepts Knuckleduster as his master to be trained by him.
Now that he is Koichi's master, Knuckleduster asks him about the speed of his Slide and Glide Quirk. Koichi states that the speed is that of a bicycle and if he goes any faster than that, he will lose control since his Quirk does not have any brakes. Knuckleduster gives Koichi protective gloves that motorcyclists use and suggests him to practice the art of falling safely. Knuckleduster also tells Koichi that he needs a new name, as "Nice Guy" will cause evildoers to make light of him. So Koichi adopts the name of "The Crawler".
During the following weeks, Knuckleduster is dedicated to training Koichi, and patrolling the streets with The Crawler and Pop☆Step to end the traffic of Trigger, which leads him to face several Instant Villains such as Akira Iwako, Teruo Unagisawa  and Bat Villain among others.
Their actions interfere in the plans of Kuin Hachisuka and the Villain Factory, responsible for the many incidents in which they are involved. Kuin wants to eliminate them, but the mysterious leader of the organization is more interested in using the Vigilantes to obtain data and blood samples from the Instant Villains.
In one of these missions, Knuckleduster finds some clues about Kuin, but not enough to find out the exact whereabouts of his brainwashed daughter. This leaves Iwao discouraged for a time, though he soon regains his spirits after crushing a few instant villains more. 
During a patrol in which Knuckleduster was absent, Koichi meets another vigilante named Stendhal, who saves his life when fighting Akira Iwako. When he returns to his penthouse, Koichi tells his master about what happened, and he stars gushing over Stendhal. Knuckleduster warns him that guys like Stendhal, since those who use "justice" as an excuse to pick fights are nothing but trouble. Koichi acknowledges that he is right about that, although he considers that Knuckleduster is not the most suitable to say something like that taking into account his way of "imparting" justice.
Turns out that Knuckleduster is right in his opinion on Stendhal because, unlike him, he does not hesitate to kill the criminals and anyone who dares stand in his way. He becomes more and more disillusioned with hero society, to the point where he begins to work with Kuin, who was supplying Trigger to several small-time villains.
One of the targeted criminals is Soga, who, after realizing the seriousness of his actions, decides to get away and remove his friends from everything related to Trigger, challenging Kuin in the process. The agent of the Villain Factory forces a fight between him and Stendhal, using a swarm of bees to inject Trigger into Soga by force, transforming him into a Porcupine-like beast to face Stendhal.
While doing his patrol, Koichi hears Soga's screams of rage. When he arrives at the place, Koichi is shocked when he sees Stendhal draw his sword and cut Soga. At that moment, Koichi realizes that Stendhal truly intends on killing Soga, so he risks his life to save the injured Soga from the murderous vigilante, being wounded in the process. Stendhal uses his Bloodcurdle Quirk to paralyze them, but as he prepares to kill them, Knuckleduster intervenes.
Stendhal recognizes Knuckleduster as the "Vigilante of Naruhata" and has no intention of fighting him because he views him as someone who is forging the path of righteousness in this crooked society and as a result they share similar ambitions. However, Knuckleduster does not view himself as being similar to Stendhal as he sees himself as a champion of justice while Stendhal is nothing more than an insane street murderer.
Stendhal is disappointed that he and Knuckleduster cannot reach a mutual understanding, but Knuckleduster knows that Stendhal is disappointed that he lost the chance to launch a surprise attack after noticing Stendhal gauge the distance between them. Then, Knuckleduster lays down a line of wire and tells the killer that he'll punch him if he steps over it. Accepting his challenge, Stendhal charges forward to strike down Knuckleduster, but when he gets close, Knuckleduster steps over the line and slams his fist into Stendhal's mask which sends the killer flying back and badly injuring his face.
Pointing out that he never said that he wouldn't cross the line himself, Knuckleduster picks up Stendhal's broken mask and tells Koichi that people like Stendhal consider themselves more powerful than they really are when they put on a disguise, acting rashly and ignoring common sense. But when they face a real challenge is when they really realize how fragile they are due to their lack of resolve.
After hearing Knuckleduster's statement and despite having his nose broken, Stendhal attacks the Vigilante again and manages to paralyze him with his Quirk. However, instead of finishing him off, a bloody Stendhal thanks Knuckleduster for enlightenment him with his "lack of resolve" speech and leaves the place. Shortly after Kazuho arrives with first aid kits to treat the wounds of Knuckleduster, Koichi and Soga.
Thanks to their activities as protectors of Naruhata, Knuckleduster, The Crawler and Pop☆Step begin to be known by the people of the city as the "Naruhata Vigilantes", although there is much misinformation about them. Their fame makes a student from Koichi's college take an interest in them, Makoto Tsukauchi, because she is writing a thesis about vigilantism, and asks Koichi to help her with her investigation.
When Koichi tells Knuckleduster about it, he warns him that Makoto's investigation may cause their identities to be revealed, and the Police Force along with Villains will come attacking in droves. Knuckleduster offers to keep Makoto silent with brute force, to which Koichi promises to deceive her as long as he does not interfere. Knuckleduster decides to let Koichi handle the situation but warns him not to let his working and private lives intertwine. After a series of incidents, Koichi manages to keep their vigilantes identities hidden thanks to a misunderstanding.
Several days later, Knuckleduster shows up at Koichi's house, where he meets Makoto and Shoko Haimawari, Koichi's mother who had come there to visit her son. Koichi and Kazuho start to freak out because they fear the reaction he may cause in Shoko and Makoto when they see the violent vigilante, but fortunately, on that occasion Knuckleduter is not wearing his Vigilante attire, and politely introduces himself to the guests as “Takeshi Kuroiwa”.
"Takeshi" gives Makoto and Shoko his business card, presented a perfect façade of a worker for a local non-profit organization, metaphorically telling them the truth that their organization is dedicated to patrolling and keeping the streets of Naruhata clean.
Makoto and Shoko become friends, and decide the next morning to tour the city forcing Koichi to accompany them. The tour proceeds without incident, but when they are preparing to return, Kuin Hachisuka decides to strike again. She injects Trigger into a cat with a Quirk, making the animal to merge with the bus that Koichi and his companions ride on. The transformed vehicle goes out of control, endangering the lives of passengers and pedestrians.
Knuckleduster notices the ruckus, and when he sees Koichi, Shoko and Mokoto in danger, he decides to take maneuvers to prevent the transformed vehicle to crash. With a makeshift cat toy, he makes the vehicle travel down a fast lane that has a destroyed overpass, but is devoid of people and obstacles, and is long enough, giving enough time for Kocihi and the Team Idaten can rescue everyone before reaching the edge.
During the rescue, he and Kazuho take care of Shoko, while Koichi helped in the operation, rediscovering his ability to jump in midair. Shoko remembers of times when an infant Koichi started using his Quirk for flight even before he could walk. She was so terrified that he would hurt himself that she punished him every time she caught him doing it until he forgot that he could. "Takeshi" mentions there comes a time when every child has to fly in some point of their lives and all they could do is teach them how to fall right. After the incident, Shoko prepares to return to her house, sure that her son is in good hands.
During all these events, Knuckleduster continued with his investigation to find the whereabouts of Kuin to rescue his daughter. At some point, he asked Soga for help. After visiting his wife at the hospital, Soga informs him of his latest clues about Kuin, which allows Iwao to advance his research.
Knuckleduster finally manages to find Kuin when she is about to release Teruo Unagisawa, a Next-Level Villain with the Eel Quirk, at the Narufest event. Koichi and Kazuho also participate along with other artists. Kuin thinks that by occupying the body of his daughter, Tamao, Knuckleduster will not dare to attack her, but she is wrong since Knuckleduster has no qualms about using his new weapon, a Taser Knuckle, against her.
Kuin tries to avoid directly facing Knuckleduster by using all kinds of tricks such as Bomb Bees, but Knuckleduster perseveres. During the fight, he receives a call from the hospital informing him of the worsening health condition of his wife. He responds that soon the whole family will be right over. Once cornered, Kuin does not hesitate to resort to more violent methods. After injecting herself with a sample collected from Teruo (who was defeated by Shota), she temporarily gains electrical powers that she channels through claws on her fingers, and manages to stop Knuckleduster’s heart in one hit.
Thinking that she has killed Knuckleduster, Kuin continues with her plan to crash the Narufest party, but Knuckleduster reappears and reveals that his Taser Knuckle also has a defibrillator function which allowed him to survive. Kuin attacks him again, but this time it is Knuckleduster who defeats her with an electric attack, using the Taser Knuckle.
Due to the Taser Knuckle's shock, Knuckleduster manages to get her swarm to leave his daughter's body by stopping his daughter's heart. He then extracts the queen bee, Kuin, from her eye, and calls back the swarm with a pheromone and bomb combination device, which explodes, killing the queen bee and the swarm.
Knuckleduster manages to resuscitate his daughter and save her, taking her to the hospital where his wife is hospitalized, who unfortunately dies shortly after their arrival.
After defeating Kuin and saving his daughter Tamao, Knuckleduster continues as a vigilante along with Koichi and Kazuo, but for a time there seem to be no more incidents related to Trigger and Villains’ imprisonment, as they’re only defeating simple petty criminals. Koichi thinks that Trigger has stopped circulating in the streets, but Knuckleduster tells him that the people behind those incidents have not surrendered.
He visits his hospitalized daughter, who has been being taken care of by Soga. When Tamao asks for her mother, he simply replies that at that moment she can not see her. Shortly after, Knuckleduster disappears without saying goodbye to Koichi, although he leaves his characteristic Brass Knuckles and his mask in his house.
After several weeks without hearing anything about him, it seems that Knuckleduster had ceased to be a vigilante after defeating Kuin and saving his daughter, but in reality, he traveled to Hong Kong, where he is still looking for clues about Trigger on his own.
After obtaining the necessary information from some dealers, he returns to Japan to visit his daughter in the hospital, and later, under his alias as Mr. Kuroiwa, he contacts detective Tanuma to give him information about Japanese importers of Trigger. Apparently, Tanuma knows Knuckleduster’s past, and he asks him if he knows anything about a villain who seems to have a similar type of Quirk as O'Clock. Knuckleduster simply says does not know who he's talking about, but advises the detective that if they meet that villain again, do not hesitate to shoot his vitals.
Soon after, he observes Koichi from a distance, happy to see that he continues to fight crime, although he has not contacted him yet.
After this, he continues to perform his duties as a vigilante without telling anyone, investigating the Villain Factory and its movements. Thanks to this, he is aware of their new agent, Number 6, who has been behind the latest incidents related to the Next Level Villains, and who is in possession of his stolen Quirk.
His inquiries and investigations lead him to deduce that the next move of the Villain Factory will be to attack the Tokyo Sky Egg event, to use it as a testing ground. He looks for what would be the ideal rooftop to provide the best view of the Sky Egg in case Number 6 decides to observe the attack, and then search for the ideal spot to snipe him.
As he had deduced, Number 6 attacks the Sky Egg with creatures known as Bombers, with the villain observing everything on a rooftop. Thanks to the intervention of the heroes, his attack fails. Seeing that he had failed and that there is nothing like that he could do, Number 6 is about to leave, but Knuckleduster is not willing to let him.
Aiming him with a sniper rifle, Knuckleduster prepares to press the trigger. Unfortunately, when he is about to shoot him, a bird passes in front of his scope, causing him to miss the shot and piercing Number 6 shoulder. Knuckleduster tries to shoot him again, but the villain quickly locates him, and despite his injury, he jumps from building to building thanks to his Quirk until he ends up on the rooftop where Knuckleduster is.
Number 6 recognizes Knuckleduster as the former hero O'Clock, whom he admires so much. For his part Knuckleduster recognizes him as the villain who is behind all the Next-Level Villains attacks and is also in possession of his former Quirk Overclock, but otherwise, he has no idea who he really is. The villain replies that he is a nobody, a thing still unformed, and he has no real name, he is just called Number 6. However, he confesses to Knuckleduster that he hopes to become O'Clock, since he admires him a lot, and he asks him to recognize him as his successor.
Knuckleduster refuses to grant his request, in fact he does not intend to allow a villain like him, who is in possession of his stolen Quirk, to continue to exist. Number 6 resigns to the fact that his idol recognizes him as a villain, and attacks him. But Knuckleduster knows all the weaknesses and drawbacks of his former Quirk, and uses them against Number 6, causing him to fall into a trap.
Knuckleduster blows up the rooftop and several floors below. Unable to maneuver in the air, Number 6 falls to the bottom, being shot several times by Knuckleduster with a shotgun while falling. Once on the bottom, the vigilante continues to beat him. Knuckleduster admits that killing him was his best long-term option, but now that he had fallen into his trap, it is worth capture him to interrogate him. Knuckleduster tells Number 6 that the building is damaged, and therefore another blast will gonna bring the whole building down, becoming his tomb. He can’t escape even with his super speed.
Number 6 tells him he is naive, because his decision not to kill him gives him the chance to strike back. Number 6 confesses to Knuckleduster that he has bomber cells in his body, as parasites, before self-detonating his left arm, blowing Knuckleduster away. Seriously injured, Number 6 praises Knuckleduster for his stubbornness, considering him as the best hero, even better than All Might himself. However, he is going to surpass him, and be the new O'Clock. Knuckleduster tells him that that will not happen, because in addition to his injuries, he will end him.
Number 6 shows Knuckleduster that, although he doesn't know as much about Overclock as he does, he can do things with the Quirk Knuckleduster could never achieve. Combining overclock with the Bomber's cells of his body, he accelerates the growth of the cells, obtaining an explosive arm where before his left arm was, then, he extends the cells to the right arm, thus obtaining two explosive arms.
Despite all this, Knuckleduster is willing to face him and attacks without contemplation. Admiring Knuckleduster's resolve, Number 6 recognizes that he is certainly the best, before attack him, initiating the final battle between the two.
After the confrontation, only the ruins of the building remain, and the result of the fight or the fate of Knuckleduster is unknown. For weeks, under the command of Detective Tanuma, the police has been investigated the place, but the only thing they could find were Knuckleduster and Number 6 blood samples. Tanuma contemplates the ruins, wondering where Iwao is and what he was up against.
Three years have passed since Knukleduster left the Naruhata Vigilantes, nor has anything been known about him since his fight against Number 6, while the villain once again made an act of presence, kidnapping Kazuho and turned her the new Kuin Hachisuka.
Koichi tried to stop her, but he was defeated, being rescued by Soga. When Koichi wakes up at Soga and his friends apartment, Soga tells him that at some point before Knukleduster left the team, he asked Soga to keep an eye on Koichi and Kazuho, and to deliver a letter to Koichi if he ever go through great difficulties.