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 illness, 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, repairing 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
The two vigilantes share a loose teacher-student relationship, with Knuckleduster helping Koichi with combat training and initially helping him use his quirk more effectively. It was Knuckleduster that suggested that Koichi change his hero name to The Crawler and pushed him to take his role as a vigilante. Despite this, the two are not especially close, with Koichi being generally terrified of Knuckleduster's predilection towards violence. However, unlike Kazuho, Koichi is able to acknowledge that although his methods are rough and seemingly excessive, Knuckleduster knows when to hold back when it comes to the force of his attacks, showing a respect for his skills as a vigilante.

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 it gets out of hand. Despite that, the two are capable of working together during Vigilante missions.

Eraserhead
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 Knuckleduster and then helps him stop Mario, Soga and his friends, who had become Impromptu Villains. Later, he warns him about the dangers of his vigilante activities.

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.

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