Iwao Oguro/Relationships

Mrs. Oguro
Iwao Oguro's wife appears to be bedridden and may be stricken with a terminal illness that has ravaged her body. Iwao appears to love his wife very much and to be remorseful over their difficult past. As she is unable to speak due to her catatonic state, it's uncertain how she feels about her husband.

Shortly after rescuing his daughter, Mrs. Oguro dies.

Tamao Oguro and Kuin Hachisuka
Unbeknownst to his fellow vigilantes, Kuin appears to be Iwao's long-lost daughter, Tamao. Flashbacks show that he and his daughter Tamao did have a rocky relationship, mostly over her interest in music, which he didn't approve of. After an argument, she leaves the house and disappears, reappearing time later as Kuin Hachisuka, who is controlling her body with her parasite bee.

After knowing this, Iwao is deeply dedicated to tracking her down, rescue his daughter and freeing her from Kuin's control. Iwao is aware that she is involved in the spread of the Trigger drug, and that she works for the Villain Factory.

He succeeds when he forcefully pulls Kuin out of Tamao's eye after a tough fight. After this, he takes her to a hospital where he takes care of her until she regains consciousness. As he is still investigating Trigger's trade, he asks Soga and his friends to watch over her bedside whenever he is out.

Her relationship with her daughter seems to have improved, although even after he saves her from the queen bee, she still gets a kick by playing the guitar because he apparently dislike it, although now Iwao does not care.

Koichi Haimawari
Iwao first meets Koichi sometime after his daughter's disappearance, before introducing to him as Knuckleduster drunkenly picking fights with others in search of a clue regarding Trigger. Koichi steps in to break up the fight, and through his kindness in trying to console the beaten up vigilante, helps to inspire Iwao to do better in himself, but bring out the best in Koichi.

Koichi formally meets Iwao when he was saved from Soga and his friends by him. Recognizing his bravery, he adopts him as a disciple and decides to train him, without bothering to hear Koichi's opinion, who at first was not interested, but in the end he accepted his offer.

The two vigilantes share a loose teacher-student relationship, with Iwao helping Koichi with combat training. Iwao does not hesitate to advise and help Koichi to refine his fighting style, how to use of his Quirk more effectively, and his Vigilante skills; and he does not hesitate to praise him when he sees his progress. Iwao methods can be rough and seemingly excessive, but Koichi does not doubt that they are of great help. It was Iwao that suggested Koichi to change his hero name to The Crawler and pushed him to take his role as a vigilante.

Iwao values Koichi a lot, to the point that he leaves his characteristic Brass Knuckles when he leaves the Naruhata Vigilantes to pursue his own goals, without saying anything to anyone. When he returns after several weeks, he proudly observes Koichi in the distance how he works as a vigilante.

When Number 6 makes himself known to Iwao, asking for his blessing to become O'Clock's successor, Iwao smugly states that he already has one, in Koichi, which greatly irritates Number 6, and eventually leads to his revenge plot to dispose of him.

After Bee☆Pop's first attack, and Koichi is rescued by Soga and his crew, he's given a letter addressed to him from Iwao. He apologizes for not being there to help and inheriting one of his problems, while also saying that he never intended to make him a "true-blue hero" because in his eyes, he already was one, his hero, and that he knows he will do whatever he can to save and help someone in need, because that is the type of person Koichi is, helping to inspire him to plan to rescue Kazuho.

Kazuho Haneyama
Like Koichi, Kazuho is a vigilantes and fellow partner of Iwao, but at first their relationship can be deemed as a rocky one as both tend to bicker with one another on more than one occasion. Kazuho does not appear to tolerate Iwao's aggressive and violent attitude, especially when he gets out of hand, while Iwao considers her a pampered and spoiled child who complains about anything.

Despite this, the two are capable of working together during Vigilante missions, and over time they begin to care about each other in situations that have nothing to do with their job as vigilantes, as it could see when Iwao encouraged Kazuho to pursue her own dreams as an professional idol.

Apparently, he is aware of Kazuho's feelings for Koichi and that she tries to hide them, and advises her to be more honest with her feelings and show them more often, although she must do it in moderation.

Eraser Head
Iwao meets Shota Aizawa while trying to make Mario Kugutsu spill the beans about Trigger. Thinking he was causing trouble with a Quirk, Shota tries to stop Iwao, but he responds violently starting a fight between the two. Once Shota realizes that he's Quirkless, he stops fighting him because if he isn't using a Quirk for illegal activities, then he falls outside hero jurisdiction.

Shota stops fighting apologizes to Iwao and then helps him stop Mario, Soga and his friends, who had become Instant Villains. Later on, he warns him about the dangers of his vigilante activities.

Later, Shota thanks Knuckleduster for his assistance though warns him about the dangers of his vigilante activities. Knuckleduster replies that Heroes need people like him to deliver justice without restrictions. Shota finds it a ridiculous notion that Knuckleduster believes his unlawful justice are crimes of conscience and finds him to be a troublesome old man but respects his integrity.

Soga Kugisaki
At first, Iwao considered Soga, along with his friends Rapt and Moyuru, a simple thug that only caused problems, but after saving his life from Stendhal, he decided to make him his ally. Iwao asked him for help in his investigation to discover the whereabouts of Kuin Hachisuka, with whom Soga had previously had contact.

Iwao relies enough on Soga that, after defeating Kuin and rescuing his daughter Tamao, he asks him and his friends to take care of his daughter while he handles other matters.

Iwao came to trust Soga so much that he even reveals things he keeps hidden from his Koichi and Kazuho, such as the warehouse he uses as a secret base. Shortly before Iwao disappeared, he asked Soga to keep an eye on Koichi and Kazuho, and handed him a letter to give to Koichi in case they ever had trouble. Soga keeps his promise, and as a sign of respect for old Vigilante, he begins to wear a trench coat similar to his.

Eizo Tanuma
Eizo is one of the detectives who investigate the Factory, and is one of the few people who know Iwao Oguro's past as the hero O'Clock. The two prominently worked together as frequent partners during his time as a Pro Hero, assisting in many cases such as the Underground Masquerade incident. They both show to demonstrate a deep respect and understanding for each other and the usefulness of each others methods.

After he loses his Quirk and becomes the vigilante Knuckleduster, Eizo remains an ally of Iwao, providing a connection between him and the official authorities. In return, Iwao provides him with vital information obtained through questionable methods to help Eizo in his investigations.

Eizo does seem to care about Iwao, as in the aftermath of his battle with Number 6, he wonders what happened to him. He also takes his advice and life lessons to heart, first when Soga throws some of his words back at him, and later when he starts to examine the similarities between the cases of O'Clock's Underground Masquerade raid, and the work of the Villain Factory.

Giran
Iwao appears to have a purely business relationship with the broker, obtaining equipment, substances and items that're sometimes less than legal to deal with difficult Quirks.

Stendhal
Iwao heard about Stendhal for the first time after he saved Koichi from an instant villain. Seeing him excited about this new vigilante, Iwao warns Koichi that there are dangerous guys who use justice to cause all kind of troubles. His warning was prescient as Stendhal did not hesitate to kill criminals.

Iwao fights against Stendhal when he tried to kill Koichi and Soga. Stendhal does not want to fight against him because he thinks that both are vigilantes looking for the same goal. Iwao completely rejects Stendhal's opinion. They may both vigilantes but whereas Iwao sees himself as a badass champion of justice, he considers that Stendhal is nothing but an insane street murderer.

Disappointed by their difference of ideologies, Stendhal attacks Iwao, but he knocks him out. Stendhal's nose was completely smashed in and he was forced to retreat, causing minor injuries on the Vigilante. However, his defeat was not a complete loss, as became enlightened after hearing Iwao's comments on resolve. Stendhal thanks him for this revelation before retreats.

Number 6
Number 6 has a great admiration for him when he was de pro Hero O'Clock, to the point he self-inflicted a scar on his face similar to his when his Quirk was stolen by AFO as a tribute. However, Iwao has no idea who he is or where he came from, except that he is a member of Villain Factory, and the guy who is in possession of his stolen Quirk Overclock. Iwao knows of his existence for the first time when Eizo Tanuma tells about the villain and how he defeated entire police squad with a Quirk similar to O'Clock. Iwao just recommends that, if the police come across the villain again, they'll shoot him down immediately.

When they finally meet in person, Iwao considers him nothing more than a villain, one that he plans to eliminate. Number 6 asks him to recognize him as his successor, O'Clock II. Iwao flatly refuses for two reasons: One, he already has a disciple, Koichi Haimawari. And two, he does not intend to allow a villain like him, who is in possession of his stolen Quirk, to continue to exist.

Faced with Iwao's refusal to recognize him as his successor, Number 6 decides to becoming O'Clock II by eliminating him, starting a bloody battle between the two. Despite having no qualms about killing him, Number 6 considers Knuckleduster the best one, much better than the Pro Heroes, even All Might.

The Rapper
Back during his time as O'Clock, Iwao took part in a sting operation where he infiltrated the Underground Masquerade to help expose their use of Quirk-related drugs. It was there where he took part in a match against Kendo Rappa, under the alias of The Rapper.

After Iwao feigns a knockout, Kendo approaches him in the med bay to give him half of the winnings plus a bonus, believing Iwao did not have his full heart in the fight so it doesn't count. He admires his skill and insists they have a full fight-to-the-death rematch, but Iwao declines. Kendo tells him his personal beliefs, that "life's no fun without a little stupid, and that it's the soul that decides hate and love", recognizing that even if Iwao doesn't admit it, he deep down loves those kind of no holds barred encounters. The two would then work together, alongside Rumi Usagiyama, to escape the arena when it was attacked by unknown assailants.

Even though he claims to dismiss his philosophy and overall attitude at first, it does appear that he has since used Kendo as an inspiration for his "Knuckleduster" persona.