Iwao Oguro/Synopsis

History
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 illegals 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 the Villain Factory, a criminal organization 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.

Vigilantes Beginnings Arc
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 stop Knuckleduster under the suspicion of being 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 KD was right in his suspicions.

It's turn 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 for the beating they suffered in their last quarrel. Then 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 oldman. 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 reunitesKoichi 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.

In one of these missions, he finds clues about Kuin.

Stendhal Arc
Later, he faces Stendhal, another vigilante who, unlike him, does not hesitate to kill the criminals and anyone who dares stand in his way. Initially, Stendhal works with the likes of vigilantes such as Knuckleduster, The Crawler, and Pop☆Step. However, he becomes more and more disillusioned with hero society, to the point where he begins to work with a Kuin, who was supplying Trigger to several small-time criminals. 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.

Stendhal intends to not only kill Soga, but also Koichi, who risked his life trying to save Soga. Stendhal uses his Bloodcurdle Quirk to paralyze them, but as he prepares to kill them, Knuckleduster intervenes. During the fight, he lays down a line of wire and tells the killer that he'll punch him if he steps over it. When Stendhal gets close, Knuckleduster steps over the line and lays him out flat, pointing out that he never said that he wouldn't cross the line himself. Stendhal's nose is completely smashed in and he’s forced to retreat despite only causing minor injuries on the Vigilante. However, his defeat is not a complete loss, as he becomes enlightened after hearing Knuckleduster's comments on resolve.

Vs. Queen Bee Arc
Knuckleduster continues with his investigation to find the whereabouts of Kuin, this time with the help of Soga, who is looking for clues for him.

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 facing directly Knuckleduster by using all kinds of tricks such as bees bombs, 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. 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.

Sky Egg Bombing Arc
After several weeks without knowing 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 his 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 the next move of the Villain Factory and its agent will be 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 injure, 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 to no 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 both.

Current Arc
At some point before this, Iwao asked Soga to keep an eye on Koichi and Kazuho, ​​and to deliver a letter to Koichi if he ever go through great difficulties.