Iwao Oguro/Relationships

Mrs. Oguro
Knuckleduster's wife appears to be bedridden and may be stricken with a terminal illness that has ravaged her body. Knuckleduster 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 is 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 Knuckleduster'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 parasyte bee.

After knowing this, Knuckleduster is deeply dedicated to tracking her down, rescue his daughter and freeing her from Kuin's control. Knuckleduster is aware that she is involved in the spread of the Trigger drug, and 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 Knuckleduster does not care.

Koichi Haimawari
Iwao first meets Koichi sometime after his daughter's disappearance, during his early days 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 Knuckleduster 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 Knuckleduster helping Koichi with combat training. Knuckleduster 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. Knuckleduster methods can be rough and seemingly excessive, but Koichi does not doubt that they are of great help. It was Knuckleduster that suggested Koichi to change his hero name to The Crawler and pushed him to take his role as a vigilante.

Knuckleduster 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 Knuckleduster. 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
The two are fellow vigilantes, but 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 Knuckleduster's aggressive and violent attitude, especially when he gets out of hand, while Knuckleduster considers her a pampered and spoiled child who complains about anything.

Despite this, the two are capable of working together during Vigilante missions. Knuckleduster also encourages her to pursue her own dreams as an 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
Knuckleduster 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 Knuckleduster, 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 apologizes to Knuckleduster 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.

Soga Kugizaki
At first, Knuckleduster 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. Knuckleduster asked him for help in his investigation to discover the whereabouts of Kuin Hachisuka, with whom Soga had previously had contact.

Knuckleduster 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.

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. Tanema is an ally of Iwao and provides 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.

Tanema seems to care about Iwao, as it could see when he was in front of the battlefield where fight betwen Iwao and Number 6 took places, wondering what happened to him.

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

Stendhal
Knuckleduster 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. Knuckleduster completely rejects Stendhal's opinión. They may both vigilantes but whereas Knuckleduster 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 Knuckleduster, 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 Knuckleduster's comments on resolve. Stendhal thanks him for this revelation before retreats.

Number 6
Number 6 has a great admiration for him, to the point he self-inflicted a scar on his face similar to his as a tribute. However, Knuckleduster has no idea who he is, except that he is a member of Villain Factory, and the guy who got his stolen Quirk. Knuckleduster knows of his existence for the first time when Eizo Tanuma tells about Number 6 (without named it) and how he defeated entire police squad with a Quirk similar to O'Clock. Knuckleduster just recommends that, if the police come across the villain again, they'll shoot him down immediately.

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 is 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.