Kai, also known as Overhaul, is a Yakuza, being part of an organization that, along with several others, ruled the underworld in the past. With the rise and prominence of heroes, in particular All Might, those organizations started crumbling down and the remnants who were not arrested were forced to live under constant surveillance. Overhaul, thus, is described as part of a "dying species".
Overhaul was found and raised by the boss of Shie Hassaikai. He views him as a father figure and believes he owes the old man a great debt for taking him in. Throughout his life, the boss recognized that Kai was troubled and he would try to hone him into an honorable Yakuza. Kai would often lash out at anyone who badmouthed the Yakuza as a child. The Boss told a young Kai that Yakuza can't hurt innocent people but still thanked Kai for trying to protect their honor. As Kai grew older, he started a venture dealing with Quirk-related drugs. The Boss told him to stay away from villainy and drugs. He understood that Kai was simply repaying the kindness after the boss saved him, but he must not go overboard nor must he stray from his path. He even refused Kai's plan to reinstate the Yakuza.
At one point, the Shie Hassaikai's leader took charge of his granddaughter, Eri. At a young age, she accidentally used her newly manifested Quirk and vanished her father from existence. Her mother believed her existence to be cursed and gave the girl up to her father. He revealed to Kai that Eri has a strange Quirk that did not resemble anything from her family lineage and believes it is a genetic mutation. The boss does not know much about how Eri's unique Quirk works but due to it resembling Kai's Quirk is the reason why he has asked him to take care of Eri and investigate her Quirk.
During an investigation, Kai starts thinking that Eri's Quirk is on a completely different level from his restoring or repairing and hypothesizes that Eri's unique Quirk can rewind genetics; Kai finds Eri's Quirk to be terrifying. Through unknown means, he discovered that her blood had properties that suppress the bodies' ability to manifest 'Quirk Factor', inhibiting someone to use his powers.
With this information, Kai brought forth his plan to the Shie Hassaikai's boss. Kai planned on selling the Quirk-destroying bullets to Villains who would use them on Pro Heroes and after a while, the Yakuza would distribute a serum that would undo the damage done and restore the lost Quirks to the Heroes. These products would be at a high price. Therefore, with the villains having the Quirk-destroying bullets, the heroes having the serum, and Eri in the middle, the Shie Hassaikai can monopolistically control the market, earning massive profit and eventually emerging from the darkness.
However, the boss had already refuted Kai's plan and questioned his morality regarding humans. The boss advised him to leave if he was going to disobey their way of thinking. Kai did not plan on going anywhere as all he wanted to do was repay the boss for taking him in and despite the boss's refusal, told him to watch from the sidelines as he went through with his plan. Kai blamed Quirks for bringing forth heroes that put the Shie Hassaikai's boss in a coma and with his plan he could restore the Yakuza back to its former glory.
After Shie Hassaikai's boss fell into a coma, Kai took over the organization and then began to experiment on Eri to turn her Quirk into a weapon. He manufactured them into bullets that be shot at a target and inject them with her blood. He did this by destroying Eri with his Quirk, collecting blood, and then repairing her. He did this hundreds upon thousands of times, despite Eri's pain. Eri tried to escape and resist many times but to no avail and eventually accepted her fate as a prisoner. However, the effect of these bullets was temporary, and Kai used further tests to be able to perfect his experiment until he had refined the process enough to create bullets that he believed could permanently destroy Quirks.
Overhaul and some of his subordinates are first seen destroying the getaway vehicle of Team Reservoir Dogs. He comments on how weird it is that a group of adults teamed up just to steal a cash register from a store, and declares that they were sick.
Overhaul enters the League of Villains' hideout but is unimpressed with Tomura and his members as well as his unclean hideout. Overhaul explains to the League of Villains that with All For One's downfall, someone will eventually become the next ruler of the underworld. However, Tomura replies that he will become the next underworld ruler after he gathers more members and crushes the current Hero society. Overhaul asks Tomura if he has a plan, to which Tomura responds that he thought Overhaul wanted to join them. Overhaul is disappointed that Tomura does not have a plan to achieve his objective and laments that he wasted the potential of Stain, Muscular, and Moonfish. Overhaul reveals that he wants the League of Villains to join him so that he can teach them the proper way to run an organization and use them to gain financial capital so that he can get one step closer to becoming the next underworld ruler.
However, Magne rejects this proposal and magnetizes Overhaul, bringing him into Magne's range. Magne hits Overhaul on the head with her weapon, to which Overhaul responds by touching Magne. Overhaul's physical contact causes Magne to blow up, much to the League of Villains' shock. Overhaul blames the League for throwing the first punch and dislikes the blood running down his face. Mr. Compress charges forward and touches Overhaul but his Quirk does not activate due to being shot in the arm. Overhaul is enraged that he has been touched and severs Mr. Compress's left arm by blowing it up. Furious, Tomura dashes to Overhaul and prepares to decay him. Overhaul calls for a human shield. Tomura touches Overhaul's subordinate instead which causes Tomura to disintegrate the subordinate.
Suddenly, Overhaul's subordinates crash in. Seeing that both of their organizations have suffered casualties with the deaths of their respective members, Overhaul ceases fighting and will talk to Tomura again after he calms down. Himiko Toga and Twice want to kill Overhaul for murdering Magne, but an equally angry Tomura stops them. As he walks out, Overhaul throws a business card at Tomura's feet and tells Tomura to call him again after he has calmed down and thought about the organization he wants to build.
The following weekend, Overhaul chases after Eri and unintentionally bumps into Izuku Midoriya and Mirio Togata. Kai apologizes to Izuku for his daughter's behavior because she gets carried away with her fun and games. Mirio tells Izuku that he forgot to put his mask on. As Mirio apologizes to Kai, Izuku is aggravated by his mistake of letting Kai know that he and Mirio know each other, which will make Sir Nighteye's job more difficult. Izuku acts innocuously to correct his mistake while Mirio identifies Kai as a member of the Shie Hassaikai due to his mask, to which Kai states not to pay heed to his mask as he simply dislikes dirt. Kai mentions that he has never seen the two of them before and wonders if they are rookies due to their youth. Mirio replies that they are indeed rookies. Kai asks Mirio the Hero office he is affiliated with and Mirio replies that they are still students participating in field training. Mirio and Izuku prepare to leave, but Eri tells Izuku not to leave as she starts shedding tears.
Izuku tells Kai that his daughter is frightened by something, to which Kai replies that he scolded her. However, Izuku is not convinced since Eri is clutching onto him tightly and suspects that there is more than meets the eye. Izuku asks about the bandages, which Kai replies that Eri falls down a lot, but Izuku is still not convinced as she is completely frightened and finds the situation to be unnatural. Kai politely asks Izuku to not impose his idea of normal on other's families while Mirio tries convincing Izuku to leave since many people have different dispositions. Izuku breaks the ice and asks Kai what he is doing to the girl.
Kai gives in and tells the heroes to follow him as his situation with his daughter is an embarrassing topic. Kai, Mirio, and Izuku holding Eri follow Kai into the alley. Kai comments that his daughter defies him all the time and finds understanding children to be quite difficult especially when it comes to considering the kind of person they want to become. Izuku and Mirio see Kai removing his glove and has killer intent. Suddenly, Eri runs to her father, causing Kai to stop removing his glove. Kai apologizes to Izuku and Mirio for Eri's tantrum and thanks them for listening to his worries. Kai wishes the heroes good luck as he leaves with Eri. Mirio stops Izuku from going after Eri while commenting that Kai used his killer intent to make Eri listen to him. Mirio asks Izuku to respects Sir Nighteye's orders because chasing Kai too far will make him harder to catch.
At his hideout, Overhaul comments on Eri's sickly behavior and asks Chronostasis to prepare a bath as his subordinate apologizes for letting Eri out of his sight. However, Overhaul is not interested in the subordinate's excuse and kills the subordinate with his Quirk, telling Chronostasis to clean up as well. Chronostasis prepares to carry out his leader's orders while Overhaul comments that all people are sick with Hero Syndrome. Overhaul wants Eri to stop being selfish as she is the key to his entire plan. Overhaul shows her an experimentation lab and asks her to no longer sully his hands.
A few days later, both Kai and Tomura meet face to face once again, this time in a room beneath the Shie Hassaikai's headquarters.
Mimic asks Tomura if what he said on the phone the other day was true; that he would side with them depending on the conditions. Tomura puts his foot on the table and states that their villain organizations' motives align; Kai wants to use the influence of the League of Villains' name while he wants to expand their influence. Kai asks Tomura to put his foot off the table, but Tomura defiantly says that he should be bowing before him in respect. Tomura tells Kai that the League of Villains do not answer to anyone and they move however they like, but that the League of Villains are willing to work with anyone in a "joint venture". Kai asks if this is the only condition, to which Tomura states there is another condition: Kai must tell him about his plan since he cannot give his organization's name for a worthless scheme. Tomura reaches into his coat for something. Suddenly, Chronostasis, Kai's right-hand man, puts his gun at Tomura's head while Mimic uses his Quirk and grabs Tomura on the head. Kai orders Chronostasis and Mimic to leave Tomura alone as he willingly came to their hideout to discuss the future. Chronostasis and Mimic obey their leader's command.
Tomura pulls out what seems to be a tranquilizing bullet and says that Mr. Compress was no longer able to use his Quirk after the bullet was fired onto him, he then asks if Kai's plan has something to do with it. Kai replies that he is going to destroy justice. Kai comments that he heard that All For One controlled others by taking their Quirks; his plan brushes up on that approach a little bit and notes that all the preparations are proceeding according to plan.
After his meeting with Kai, Tomura heads back to his League of Villains hideout and informed his members that they had allied with the Shie Hassaikai, sealing the deal by having Himiko Toga and Twice work for them. At first, both are in disagreement with this alliance, because they want revenge for the death of Magne, but Tomura manages to convince them to collaborate.
After Tomura convinced Himiko and Twice to temporarily become part of Kai's Shie Hassaikai, they go to meet with Kai himself. Kai apologized about Magne as he had not wanted to kill "him" either and understood that they held a grudge against him but as long as they were working together, he asked for their complete cooperation so that the plan could proceed smoothly. Kai wanted them to obey his orders and his first order being them to explain their Quirks in order to facilitate better coordination. At first, both refused to respond, but Shin Nemoto used his Quirk to force them to tell the whole truth about their powers.
Shin asked Himiko and Twice if Tomura has plans on betraying them. Himiko and Twice replied that Tomura will not betray them. Kai welcomed Himiko and Twice to the family and due to them being wanted criminals, he could not let them do as they pleased and ordered them not to leave the underground residential area until they had been given orders. Kai walked away and told Himiko and Twice that once they have given him reason to trust them, then they could do as they please.
A few days later, after a meeting regarding the Shie Hassaikai, the heroes have decided to raid theirs headquarters in order to save Eri. Inside his headquarters, Kai notes that the heroes have come. Inside the headquarters in one of the rooms, Overhaul apologizes to the previous Yakuza Boss about the noise that is going to unfold. It is 8:30 A.M. and the Heroes and Police Force are outside the Shie Hassaikai Headquarters and commence the operation. While the members of the Yakuza and Himiko and Twice try to stop the raid, Kai tries to flee the underground routes, taking Eri with her, being escorted by Chronostasis, Shin and Deidoro Sakaki. But within minutes of starting the raid, Mirio went off on his own and manages to catch up Kai.
Kai is surprised that Mirio was able to get to them so easily; Mirio replies that he took a shortcut and is here to take Eri into custody. However, Kai is adamant that Eri doesn't want to be in Mirio's custody and mocks Mirio for his futile heroics, to which Mirio replies that he came to rectify his mistake. Seeing that there is no point in persuading Mirio, Kai begins to walk away along with Chronostasis. Mirio charges towards Kai as the Shie Hassaikai's leader tells Mirio to die. Suddenly, Mirio's sense of balance becomes hazy and he falls onto the ground. Mirio's unbalance is due to Deidoro, a member of the Eight Expendables. Mirio realizes that Deidoro's Quirk is making him unbalanced as another of the Eight Expendables, Shin, fires his gun at Mirio. However, Shin's bullets phase through Mirio. Shin asks Mirio about his Quirk, to which Mirio reveals his Quirk's ability. However, Mirio did not actually want to reveal his Quirk to the enemy which surprises him.
Deidoro criticizes Shin for missing, causing Shin to knock Deidoro's beer bottle away from him in retaliation, reminding him to support him. Mirio realizes that Shin's Quirk lets him force people to talk. Shin fires at Mirio again as Mirio's body phases the bullets; Shin comments that he is different from the other Eight Expendables as only he is privy to Kai's ambitions, although Mirio rhetorically questions Kai's abuse of his daughter. Shin replies that compassion is unneeded for progress to smoothly flow and Mirio should understand as well, referring to him overlooking the incident when he first encountered Eri, to which Mirio says that he is correct, which shocks Mirio as he didn't want to say that. Shin's Quirk, Spitting the Truth, forces his opponent to answer his questions with their true feelings. Before he was scouted by the Shie Hassaikai, Shin was a con man with repeated offenses. Shin knows all too well that people hide their true natures which are their weaknesses and at times, he will draw out their weaknesses that even his victims were unaware of.
Shin questions Mirio's motives for saving Eri, wondering if he only wants to save her to make himself feel better. Mirio has stopped in his tracks. Seeing that Kai is leaving, Shin decides to finish off Mirio so that he can quickly rejoin Kai. Shin fires his bullets and Deidoro throws his knives at Mirio. Suddenly, Mirio uses Phantom Menace, a move in which he phases himself and quickly strikes down his opponents, knocking down both Shin and Deidoro. Mirio agrees with Shin that he let Eri go back into despair and already knew that he carried a sin for doing so. Mirio is a man who grew to accept his weaknesses and was able to strive to achieve his goals because he acknowledged his weaknesses; the same holds true about his heart in that his weaknesses were not something to hide. Deidoro is surprised that Mirio could move well under the effects of his Quirk, although Mirio is able to withstand Deidoro's Quirk effect thanks to putting up with many sensations on a daily basis. Mirio cannot stand to see Eri without a smile on her face and finds that to be unforgivable. Mirio quickly strikes Deidoro and Shin again and charges towards Kai.
Mirio appears before Kai and attacks simultaneously. Mirio strikes Kai with a backhanded smack, which Kai avoids, but he still managed to leave a mark on Kai's face while kicking Chronostasis. Mirio avoids hitting Eri thanks to his Permeation and strikes Chronostasis down with his kick. Chronostasis falls back as Mirio grabs hold of Eri. Eri tells Mirio to run away otherwise Kai will kill him. However, Mirio promises to never let Eri be sad again and tells her that everything is fine because he is her Hero.
Mirio Togata has recovered Eri from Kai. Kai begins wiping off the mark Mirio left on his face as he threatens Eri to return to him because she was born to destroy people, otherwise Mirio will be murdered. Before Eri can speak, Mirio tells her to not listen to Kai's words. Kai becomes annoyed that Eri's selfishness has caused him the need to get his hands dirty. Kai mocks Eri for her existence being a curse, much to Mirio's chagrin who wonders why Kai is saying such harsh words to his own daughter. However, as he takes off his glove, Kai reveals that Eri is not his daughter. Kai places his hand on the ground and deconstructs it using his Quirk.
Kai reconstructs the crumbled ground into massive spikes that attack Mirio, but Mirio phases his body while holding up Eri with his arms, protecting both of them. Mirio is surprised at Kai's quick speed as well as his lack of regard for Eri while Kai himself praises Mirio for being more than just his Quirk. Kai launches more spikes at Mirio, uncaring at Eri's safety since he can just "repair" her easily and reveals to Mirio that Eri has experienced it firsthand; Kai's words enrage Mirio. Mirio continues avoiding the spikes while Kai asks Mirio about Eri's safety in his hands since if she gets hurt she cannot be healed immediately and that having her is making it troublesome for him as he cannot run away using his Permeation while carrying Eri. Due to him reconstructing the area with spikes, Kai has managed to cut off Mirio's escape route. Chronostasis has awakened and has a gun in his hand that is loaded with Quirk-disabling bullets. Mirio is surprised that Chronostasis is still awake after being hit by him and assumes that Deidoro Sakaki's Quirk made his physical attacks weaker at the time. Kai orders Chronostasis to aim at Mirio's arms while he destroys the spikes so that Mirio loses his ground. Kai deconstructs the ground, causing Mirio to be in mid-air. Mirio conceals himself and Eri using his cape; Chronostasis fires at Mirio but misses due to his target being concealed by the cape.
Suddenly, Kai realizes that Mirio will appear before Chronostasis and begins reconstructing the ground beneath his second in command. Mirio does indeed appear before Chronostasis, using his Quirk's instant transportation to do so. Mirio's shoulder knocks Chronostasis's arm which causes him to drop the gun and is about to hit Chronostasis with a punch, but Kai launches Chronostasis away from Mirio by reconstructing a platform which saves Chronostasis, who apologizes to Kai. Seeing that Mirio is stronger than he thought, Kai decides to break Mirio's morale by killing Eri. Before he can do so, Mirio uses his instant transportation to appear behind Kai. Kai realizes this and tries blocking Mirio with his hand, but Mirio phases his arm through Kai and punches him in the face.
Kai lands on the ground as Mirio states that his cape is for bundling up Eri who is in terrible pain. Chronostasis gets up and tries getting the gun, only for Mirio to appear before him and knock him back down. Mirio praises Kai for his abilities being far above that of the average Yakuza but tells him that he is far stronger, using his instant teleportation to appear before Kai and lands another punch against him. Eri watches the fight and is surprised at not only Mirio's strength but the lengths he is willing to go for her. Mirio swears to never let Kai lay another finger in Eri and promises to take them both down, declaring that he has them in checkmate.
Kai becomes enraged at Mirio calling out his name since he abandoned it long ago. Years ago during Kai's childhood, the boss of the Shie Hassaikai took Kai in since he had nowhere he could call home and asked for Kai's name, to which he states that his full name is Kai Chisaki. Several years later, the boss of the Shie Hassaikai lamented about the destruction of a Yakuza group; Kai told the boss about the plan he had discussed with him before, but the boss did not take heed of it because falling off from the part of humanity would cause his demise as a gangster since people would not follow brutes with no hearts.
Without a doubt, Mirio had defeated Shin by overwhelming him with three strikes to the head as well as removing the bullets from his gun. For ten minutes Shin's mind was clouded and he could not move. However, Shin is able to maintain consciousness and continue crawling because of his undying devotion to Kai. In the past, whenever Shin questioned someone about their true nature, they would always lie to him and whenever he asked why they lied, they became flustered and distanced themselves from him which made it difficult for Shin to makes friends as all he wanted was one friend he could trust. That one friend Shin wanted trust from was none other than Kai.
Because of Kai being the only person to be honest with him, Shin became utterly devoted to Kai. Shin continues crawling towards Kai with tears rolling down his right eye. Kai sees Shin as well as Mirio. Kai immediately throws a case of Quirk-disabling bullets at him. Shin opens the case and loads one of the bullets in his gun. Kai orders Shin to open fire as Shin aims his gun at Mirio.
As he throws the Quirk-destroying bullets to Shin, Kai thinks about the lengths his organization had to go through in order to create the lethal weapon; originally, the effect of the element emitted from Eri's body would only cause a victim to lose their Quirk for a day or two and they would naturally recover. In order to obtain a complete product, they needed to figure out how to permanently preserve the element's effect which they eventually succeeded. However, due to money and time constraints, Kai could only make five Quirk destroying bullets. The reason Kai took action in making these Quirk destroying bullets is because of All For One's downfall.
Shin grabs the case of Quirk destroying bullets and Kai orders him to fire. Only Shin knows the value of the Quirk destroying bullets and cannot waste a single shot; Shin tries to figure out a way to make Mirio deactivate his Quirk. Suddenly, Shin realizes that Mirio will do whatever it takes to protect Eri, so he aims his gun at Eri. Mirio immediately jumps towards Eri. Mirio was able to tell that Eri was endangered because she wouldn't scream or cry whenever murderous intent was directed at her and she would simply close her eyes, be silent and grit her teeth; this was Eri's habit that was engraved in her after living in pain and fear for a long time.
Mirio has reached Eri while Shin fires the Quirk destroying bullet. Eri opens her eyes and sees a smiling Mirio who is about to be hit by the Quirk-destroying bullet. Mirio thinks about his childhood where his family's Quirk made it difficult for them to become Heroes, which is why his father gave up on becoming a hero. However, Mirio refused to waver and throughout his life he worked extremely hard to hone his Permeation; eventually, Sir Nighteye saw potential within Mirio and made him his sidekick. Under Sir Nighteye's guidance, Mirio was able to turn his Permeation into a fully combat Quirk; during this time Mirio along with Tamaki Amajiki and Nejire Hado began being acknowledged as the Big Three.
Mirio covers Eri as the Quirk destroying bullet is shot into his body which begins destroying Mirio's Permeation. Kai states that due to Quirks people are entitled to dream and begin thinking that they become people that matter, to which Kai believes that these people are sick in the head. As Shin celebrates his triumph, Kai laughs at Mirio's efforts and mocks Mirio for smiling because everything he has worked so hard to cultivate has been reduced to naught. As Kai is about to deconstruct the ground, Chronostasis is thrown at him, courtesy of Mirio's kick. Mirio appears before Kai and punches his arm. Mirio tells Kai that all his hard work isn't in vain because he will still be Lemillion. Kai tries attacking Mirio but Mirio is still able to make his body intangible, which causes Kai to miss. Shin is shocked that Mirio's Quirk hasn't completely been destroyed yet.
Fifteen minutes have passed since Mirio caught up to Kai and now, 5 minutes have passed since Mirio was shot by the Quirk destroying bullet and he continues fighting Kai. Eventually, Mirio loses his ability to use his Permeation. Mirio fights one-third of the battle in a Quirkless condition which allows Kai to finally damage Mirio. Mirio is hurt and damaged by the spikes Kai created. Despite being greatly damaged himself, Kai mocks Mirio for wanting to be a hero and to try and save Eri; Kai blames that delusional thinking for being the problem with today's society and will cure everyone using Eri's power. Kai is about to pierce a spike through Mirio, but Mirio yells out Kai's name, much to Kai's chagrin.
Suddenly, Izuku comes smashing in. Izuku sees Kai and delivers a One For All: Full Cowl punch to his arm. Izuku's enhanced punch sends Kai flying back; Shota tells Sir Nighteye to secure Eri. Sir Nighteye is amazed that Mirio Togata has been fighting all by himself against the boss of the Shie Hassaikai and his closest aides by himself and not only managed to defeat the aides as well as injuring Kai but on top of all that, managed to rescue Eri. Sir Nighteye grabs hold of Mirio and Eri. A proud Sir Nighteye praises and congratulates Mirio for everything that he has done.
Izuku and Shota charge at the injured Kai; Kai tries using his Quirk but to no avail as Shota has disabled it with his Quirk. Cornered with nowhere to run, a distressed Kai orders Chronostasis to wake up. Shota notices something approaching them and pushes Izuku aside; Shota's shoulder is struck by a minute hand that is emanating from Chronostasis's head. Chronostasis reveals that his Quirk allows him to manifest a minute hand and anyone struck by it will have their movements slowed. Kai is glad that Chronostasis has woken up; he was able to heal Chronostasis when Mirio kicked Chronostasis towards him and during that moment he fixed his head. Shota is forced to blink due to the cut from Chronostasis, which allows Kai to use his Quirk again. Kai belittles the Heroes' efforts as he constructs massive spikes from the ground, which stalls Izuku and Shota.
Kai refuses to allow his plans to be ruined by the heroes. Kai knows that Shin Nemoto also wants him to succeed and approaches him. Kai thanks Shin for everything he has done for him and knows fully well that he will gladly die for his sake. Kai uses his Quirk and destroys both himself and Shin. Kai and Shin's deconstructed bodies reconstruct and fuse together with Kai, much to Izuku's horror.
Kai admits that Mirio was stronger than him but his efforts will all be in vain. The fused and transformed Kai appears, looking monstrous and grotesque with sharp bird-like arms from his shoulder blades and his original arms becoming larger with sharp claws. The transformed Kai declares that Eri will be returning to him.
Having fused with Shin, Kai feels relieved. Izuku analyzes the situation; Sir Nighteye is protecting Mirio and Eri, Shota has disappeared and the transformed Kai has been healed. Kai mocks Mirio for wasting his time trying to become a Hero as it has been all for naught now that he has lost his Quirk and could have avoided this tragic fate had he simply not got involved. Izuku and Sir Nighteye are surprised that Mirio has lost his Quirk. Kai continues bashing Mirio for continuing his Heroics even after losing his Quirk as it has now dragged his peers into his mess.
Kai charges forward while Izuku appears above and throws a column of rock at Kai who destroys it with ease. Izuku grabs another column of rock and tries again only for Kai to smash it to pieces and counters by reconstructing the broken pieces into sharp stones. However, Izuku's iron soles protect him from the sharp edges and he proceeds to smash Kai's constructed sharp stones into pieces. Seeing Izuku's simple battle style, Kai is not impressed. Suddenly, Kai is hit by one of Sir Nighteye's Hypermass Seals but manages to block it; Sir Nighteye orders Izuku to protect Mirio and Eri while he handles Kai. Izuku goes to Mirio and Eri's location with his enhanced speed as he remembers Shota telling him to not do anything that will leave him unable to move.
Sir Nighteye asks about Shota's whereabouts, to which Kai reveals that he took an interest in Shota's Erasure Quirk and his subordinates are escorting him to the meeting room, causing Sir Nighteye to wonder if Kai is afraid of having his own Quirk destroyed. Sir Nighteye assumes that Kai has completed his Quirk Destroying bullets and used one on Mirio. Sir Nighteye mocks Kai for being afraid of Mirio's power as Kai creates more arms for himself to attack with using his Quirk. Kai attacks but Sir Nighteye is able to evade his attacks. Kai sees that Sir Nighteye's movements are familiar and assumes that he is Mirio's teacher. Sir Nighteye thinks back to the lessons he taught Mirio and is happy as well as proud that he took his teachings to heart. In return, Sir Nighteye vows to keep him and the person whom he wants to protect out of harms way.
Izuku reaches Mirio and Eri and sees that they are fine. Izuku smashes open the wall which reveals the path he along with Shota and Sir Nighteye took to get here. Izuku decides that they should put distance between themselves and Kai. Izuku carries Mirio and Eri into the pathway; Eri apologizes while Mirio looks back to see Sir Nighteye.
During the battle between him and Kai, Sir Nighteye activated his Foresight Quirk. Whatever future he sees cannot be changed; this is the absolute rule as he himself has tried to change the future to no avail. Ever since he saw All Might's death, Sir Nighteye decided to never read people's futures and would only looks seconds and minutes into the future from then on. Sir Nighteye did everything in his power during the battle to deny Kai the future that was in his grasp and even though his actions were drawing out the inevitable this was the best course of action he could take. Unfortunately, Sir Nighteye reached his limit and was impaled by Kai's reconstructed rock spikes. Having touched Kai, Sir Nighteye saw into his future; amidst the scene of his likely defeat, Sir Nighteye held onto a silver of hope that Eri would be saved, the boys safe and Kai defeated as well as imprisoned.
Kai goes past the impaled Sir Nighteye and chases after Izuku. Izuku is horrified at Sir Nighteye's terrible condition as Kai launches rock spikes at Izuku. Izuku stomps on the ground using his enhanced might, preventing Kai from reconstructing the ground. Kai sees through Izuku's plan but is surprised that Izuku is displaying greater strength than a few minutes ago. The reason for Izuku's enhanced might is because he has activated One For All 20%, even though its making him feel great pain.
Kai berates Izuku for joining the fight and that he will end up dying. However, Izuku refuses to let anyone die even if that's what fate decided. Using One For All 100%, Izuku charges at Kai while declaring that he will bend the future.
Izuku's attack defeated Kai who fell to the ground defeated and unconscious.
He was subsequently arrested and was to be transported to a villain's hospital but the police escort was attacked by League of Villains. Despite the intervention of a hero, the efforts of the villains prevailed and they proceeded to retrieve Kai. He was unfazed by their arrival but he got surprised when Mr. Compress compressed part of his arm. He got angry when Tomura stole his Quirk drug however, his rage turned to fear as Tomura decayed and cut off other arm. Tomura then sadistically taunted the now helpless and powerless Kai, at how he wanted others to lose their Quirks but now he himself is one of them. Kai was left stunned at this before screaming in frustration at this turn of events, as he watched Tomura make off with his creation.
A news report later detailed the attack on the police escort but no information was given on Kai's status.