Kai Chisaki/Synopsis

History
At some point in his childhood, Kai lived at an unnamed orphanage with other children. It is here where he read a book concerning an old theory; that the Quirk phenomenon was caused by a pathogen carried by mice. While the theory lacked any hard evidence, this caused a young Kai to believe that all Quirks are a disease people are afflicted with, a belief that would remain with him to his adulthood.

Kai Chisaki 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, Overhaul 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; Overhaul 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, choose to put him in a coma and told him to watch from the sidelines as he went through with his plan, intending to wake his boss up once he finished his plan. Kai blamed Quirks for bringing forth heroes that pushed the boss in into a corner and with his plan he could restore the Yakuza back to its former glory.

After putting the Shie Hassaikai's boss into his coma, Kai took over the organization and then began to experiment on Eri to turn her Quirk into a weapon. He soon manufactured them into bullets that can be shot at a target to 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 Overhaul 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.

Provisional Hero License Exam Arc
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.

Shie Hassaikai Arc
Overhaul is later approached by Twice. After talking with one another, Overhaul asks Twice to meet Tomura Shigaraki.

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. Overhaul 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 Overhaul 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 Overhaul as a member of the Shie Hassaikai due to his mask, to which Overhaul 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. Overhaul 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 Overhaul 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 Overhaul 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. Overhaul 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 Overhaul what he is doing to the girl.

Overhaul gives in and tells the heroes to follow him as his situation with his daughter is an embarrassing topic. Overhaul, Mirio, and Izuku holding Eri follow Overhaul 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 Overhaul removing his glove and has killer intent.

Suddenly, Eri runs to her father, causing Overhaul to stop removing his glove. He 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 Overhaul used his killer intent to make Eri listen to him. Mirio asks Izuku to respects Sir Nighteye's orders because chasing Overhaul 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. Then Mimic arrives with a phone telling him that Tomura wants to give his answer to his proposal.

A few days later, both Overhaul 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; Overhaul wants to use the influence of the League of Villains' name while he wants to expand their influence. Overhaul asks Tomura to put his foot off the table, but Tomura defiantly says that he should be bowing before him in respect. Tomura tells Overhaul 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".

Overhaul asks if this is the only condition, to which Tomura states there is another condition: Overhaul 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, Overhaul'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 Overhaul's plan has something to do with it. Kai replies that he is going to destroy justice. Overhaul 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 explaining his plan, Overhaul introduced Tomura to shogi, since he can learn something important from it. Although Tomura did not like the game, Overhaul told Tomura that the fun aspect of shogi is using the pieces one takes from their opponent. Using shogi as an example, Overhaul offered Tomura a deal: he would temporarily take Twice and Himiko or Kurogiri because he did not want them running around as they please. Tomura did not like the idea as they are the cornerstone of his League of Villains, but Overhaul explains that his request is a payment for having explained his plan, and also this trade can help them build trust between the two factions.

After his meeting with Overhaul, 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 Overhaul's Shie Hassaikai, they go to meet with Overhaul himself. He 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. Overhaul 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. Overhaul walked away and told Himiko and Twice that once they have given him a 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 their headquarters in order to save Eri. Inside his headquarters, Overhaul notes that the heroes have come. 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, Overhaul 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 Overhaul.

Overhaul 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, Overhaul 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, Overhaul begins to walk away along with Chronostasis. Mirio charges towards Overhaul 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 imbalance is due to Deidoro, a member of the Eight Bullets. Mirio realizes that Deidoro's Quirk is making him unbalanced as another of the Eight Bullets, Shin, appears to shot him. While Mirio confronts the two Yakuzas, Overhaul leaves the place quietly, certain that Shin and Deidoro will manage to kill Mirio. Although not without difficulties, Mirio manages to defeat the two and charges towards Overhaul.

Mirio appears before Overhaul and attacks simultaneously. Mirio strikes Overhaul with a backhanded smack, which Overhaul avoids, but he still managed to leave a mark on Overhaul'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.

Overhaul 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. Overhaul becomes annoyed that Eri's selfishness has caused him the need to get his hands dirty. Overhaul mocks Eri for her existence being a curse, much to Mirio's chagrin who wonders why Overhaul is saying such harsh words to his own daughter. However, as he takes off his glove, Overhaul reveals that Eri is not his daughter. Overhaul places his hand on the ground and deconstructs it using his Quirk.

Overhaul 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 Overhaul's quick speed as well as his lack of regard for Eri while Overhaul himself praises Mirio for being more than just his Quirk. Overhaul 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 Overhaul 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. Overhaul orders Chronostasis to aim at Mirio's arms while he destroys the spikes so that Mirio loses his ground. Overhaul 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, Overhaul 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 Overhaul launches Chronostasis away from Mirio by reconstructing a platform which saves Chronostasis, who apologizes to his leader.

Seeing that Mirio is stronger than he thought, Overhaul decides to break Mirio's morale by killing Eri. Before he can do so, Mirio uses his instant transportation to appear behind Him. Overhaul realizes this and tries blocking Mirio with his hand, but Mirio phases his arm and punches him in the face.

Overhaul 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 Overhaul 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 Overhaul 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.

Overhaul 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 his 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.

At that moment, Shin reappears beaten and battered, but ready to support his master. Overhaul immediately throws a case of the Quirk-Destroying Bullets. Shin opens the case and loads one of the bullets in his gun. Overhaul orders Shin to open fire as Shin aims his gun at Mirio. However, 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 and shoots. Mirio immediately jumps towards Eri, allows himself to be shot to protect her. When the bullet hits him, the drug immediately begins to take effect and destroys his Quirk.

As Shin celebrates his triumph, Overhaul taunts Mirio because everything he has worked so hard to cultivate has been reduced to naught, insulting him for being a diseased man with foolish dreams that are lost now. As Overhaul is about to deconstruct the ground to attack him again, Mirio throws Chronostasis's body at him to distract him to immediately appears in front of Overhaul and breaks his arm with a single punch. Mirio tells Kai that despite losing his quirk, all his hard work isn't in vain because he will still be Lemillion, and continues protecting Eri.

For five minutes, Mirio continues to fight bravely against Overhaul, but having lost his Quirk, he can no longer protect himself from attacks by the leader of Shie Hassaikai and ends up badly injured by the spikes Overhaul created. Despite being greatly damaged himself, Overhaul mocks Mirio for wanting to be a hero and to try and save Eri; he blames that delusional thinking for being the problem with today's society and will cure everyone using Eri's power. Overhaul is about to pierce a spike through Mirio, but Mirio yells out Kai's name, much to Overhaul's chagrin.

Suddenly, Izuku comes smashing in. Izuku sees Overhaul and delivers a punch to his arm. Izuku's enhanced punch sends Overhaul flying back; Shota tells Sir Nighteye to secure Eri. Izuku and Shota charge at the injured Overhaul; he tries using his Quirk but to no avail as Shota has disabled it with his Quirk. Cornered with nowhere to run, a distressed Overhaul orders Chronostasis to wake up. During the earlier skirmish with Lemillion, Overhaul healed Chrono's head injuries after the hero threw him.

Eraser Head anticipates the attack and pushes Deku out of the way. Chrono's Quirk activates, extending his hair to a point that cuts open Eraser's arm. As a result of Chronostasis's Quirk, Aizawa's movements are slowed down significantly. Shota can't stop himself from blinking and Overhaul's Quirk returns. Overhaul belittles the Heroes' efforts as he constructs massive spikes from the ground, which stalls Izuku and Shota.

With his attack, Overhaul also attracts Shin's unconscious body to him. The leader of the Shie Hassaikai refuses to allow his plans to be ruined by the heroes. Overhaul knows that Shin Nemoto also wants him to succeed and approaches him. He thanks Shin for everything he has done for him and knows fully well that he will gladly die for his sake. He uses his Quirk and destroys both himself and Shin. Overhaul and Shin's deconstructed bodies reconstruct and fuse together, much to everybody's horror.

Overhaul admits that Mirio was stronger than him but his efforts will all be in vain. The fused and transformed Overhaul 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 Overhaul declares that Eri will be returning to him.

Having fused with Shin, Overhaul 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.

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 Overhaul, Sir Nighteye activates his Foresight Quirk. He does everything in his power to deny Overhaul the future that was in his grasp and even though his actions are drawing out the inevitable, this is the best course of action he can take. Unfortunately, Sir Nighteye reaches his limit and is impaled by Kai's reconstructed rock spikes.

Having touched Kai, Sir Nighteye sees into his future; amidst the scene of his likely defeat, Sir Nighteye held onto a sliver of hope that Eri would be saved, the boys safe and Kai defeated as well as imprisoned. However, a good future doesn't exist and he can only see himself and Deku dying at the hands of Overhaul, who manages to escape with Eri.

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 it's making him feel great pain.

Chisaki berates Izuku for joining the fight and says that he will end up dying. However, Izuku refuses to let anyone die even if that's what fate decided. Using One For All 20%, Izuku charges at Chisaki while declaring that he will bend the future and save Eri.

Deku is narrowly able to avoid Overhaul and tries to deal a blow so devastating that the villain won't be able to recover from it. Aimed to be one blow to the top of the head, Deku unleashes his Manchester Smash. However, Chisaki dodges Izuku's attack and claims that compared to the Lemillion and Nighteye, Izuku is easy to predict. Izuku tries jumping out of harm's way, but is hit by Chisaki's spikes attack.

Overhaul uses his Quirk to heal himself. He complains that despite his Quirk allows him to repair his injuries and fractures, it is still a painful process. Izuku has survived Overhaul's counterattack, albeit with piercing injuries, impaled by small stone pieces in his right arm and left leg. Izuku is glad that he smashed the ground before since it reduced Chisaki's firepower.

Tired of Izuku's persistence, a mouth appears on one Chisaki's hands and uses Nemoto's Quirk to call out Eri, asking her if she wants people to die because of her. Haunted by her past with Overhaul, she goes back to the battlefield and says she doesn't want anyone to die. Overhaul forces Eri to admit she doesn't think Deku can win on his own. Chisaki then asks her what she must do to resolve this situation, to which Eri replies that she must return to him and in exchange, Chisaki must fix everything back to normal.

Overhaul says the faint hope created by Lemillion is gone. He tells Deku that Eri doesn't want to be saved. Even a barely conscious Nighteye believes that he can't win because the future dictates that he will be killed by Overhaul. Despite this, Izuku gets up and yells that he refuses to let anyone die and claims he'll save Eri no matter what, reinforcing his determination by pulling out one of the spikes and smashing it.

Suddenly, the ceiling begins collapsing and Ryukyu, Uravity, and Froppy come dropping in, which shocks Izuku. The impact shock wave knocks Overhaul down. It turns out that Ryukyu's team was tricked by Himiko and Twice to get to where Overhaul was, sending a clone of Mr. Compress to snatch Eri. Izuku tells Ochaco that he is leaving Sir Nighteye to her as he goes to rescue Eri. However, Chisaki prevents Izuku from reaching her with a rock pillar and he grabs her. Chisaki is annoyed with everything being ruined and seeing the hole, an opportunity to escape, prepares his escape by reconstructing a large and long rock pillar to take him to the surface. Izuku refuses to let Chisaki get away.

As Overhaul and Eri head for the surface, Mirio Togata's cape is caught by debris, which Chisaki finds sickening. By instinct, Eri grabs Mirio's cape after remembering Mirio's kind words, and suddenly awakens her power. Eri touches Overhaul and rewinds his body to before he fused with Nemoto, separating them both. Desperate to escape, Eri leaps into Deku's arms. Determined to rescue Eri, Deku jumps through the air and reaches out to her until she finally leaps into his kind arms. He promises to never let her go again.

Infuriated, Kai Chisaki demands that Izuku hand Eri back to him and charges at him. Izuku isn't able to move well suspended in mid-air. However, Izuku vows to not let go of Eri no matter what and remembers Mirio's question of what Hero he wants to be; he wants to be a Hero that wipes everyone's worries aways, always wins and saves everyone.

Filled with utter determination, Izuku unconsciously activates One For All 100% and prepares to attack Chisaki with a kick, while Chisaki launches reconstructed stone pillars at Izuku. As Chisaki's stone pillars begin to ensnare him, Izuku disappears and the ceiling above them collapses as a shockwave ensues. Overhaul gets up, injured of the shockwave caused by Izuku. Obsessed with seeing his plan through until the end, Overhaul heals his injuries and uses his Quirk to merge his own body with Rikiya Katsukame's.

On the surface, having used the One For All at full power and checking that not only does he have any broken bones but also that all his injuries have disappeared, Izuku realizes the true nature of Eri's quirk. However, as soon as Eri lets go of him, Deku feels the repercussions.

After fusing with Rikiya, Overhaul rises out of the cavern in a monstrous kaiju-like form and announces that Eri can rewind humans and depending on how she controls it, she could revert a human back to a monkey or even to nothingness, although she can't control her power. Overhaul calls her Quirk a curse and warns Izuku that Eri's Quirk will annihilate him if he continues to hold onto her and demands he hand her over because only with his Quirk can he stop Eri's cursed Quirk.

Deku refuses and claims Eri's Quirk is an incredibly kind one, one that healed him. He understands how Eri's Quirk affects his body and believes if he accumulates injuries at an even greater speed, then he won't be rewound out of existence. Instead, Eri's Quirk allows Deku to access One For All at 100% infinitely.

Overhaul makes wild claims that Deku and Eri don't understand the importance of her power. He researched extensively to develop the Quirk-Destroying Bullets and maximized Eri's Quirk's effectiveness to achieve his goal of eradicating Quirks. Overhaul believes Eri's curse can return people to the normalcy humans had before the Quirk phenomenon. Irate, the gigantic eight-armed monster attacks by reshaping the land into giant lances and thrusting them toward Deku.

However, Izuku's greatly enhanced speed thanks to One For All: Full Cowl: 100% enables him to dodge with ease and positioning under Overhaul. Izuku delivers a kick packed with massive strength and sends the monstrous villain reeling into the air. The powerful blow causes Overhaul to lose consciousness momentarily, but soon recovers it.

Infuriated that no one understands that he's trying to change the world for the better, Overhaul goes right back at the young hero and transforms his giant body into a typhoon of mass. Overhaul believes that people like the boss and Izuku never see the larger picture. He demands Izuku stay out of the way and tries to attack again, but Deku is far too fast.

Izuku jumps towards the airborne Kai who has healed his injuries by reconstructing himself and slashes at Izuku with one of his claws. However, Izuku evades with ease and knows that Kai can repair himself with his Quirk through disassembling and reassembling at a fast rate, but he plans on using the great power of One For All: Full Cowl: 100% to close that gap and beat him before he gets the chance to recover with his Quirk.

While Izuku questions how he can call himself a hero if he can't save one small girl right in front of him, Izuku charges towards Overhaul and starts pummeling his monstrous body with a powerful barrage of punches, shattering it as Overhaul is unable to counterattack. Izuku reaches Overhaul's own body and finishes off his opponent with a powerful smash at 100% directly to his face, that strike Overhaul's mask off.

Before they hit the ground, Deku grabs one of Overhaul's giant limbs and throws the entire monster into the ground, defeating him. As he lands on the ground, Izuku asks Eri is she is fine. Suddenly, Eri's Quirk starts going out of control.

Barely conscious for the beating, Chisaki remembers getting into a fight when he was younger because someone said the Yakuza were accursed villains. The boss told a young Kai that the Yakuza couldn't harm honest folks. Even so, the Boss thanked Chisaki for trying to protect the Yakuza's honor. Frighteningly impassioned, Overhaul rises up and creates a giant hand that tries to crush Deku, but Eri's power explodes into a wave that rewinds Overhaul all the way back to his original form, separating him from Rikiya, who also recovers his original body.

Uravity quickly secures his hands and restrains him, but she begins to worry about Deku when she sees his life in danger due to Eri's uncontrolled power. Fortunately, Shota is taken to the scene and manages to save Izuku by using his Erasure on Eri, stopping her Quirk. With the situation under control, the Police Force and Heroes search the Shie Hassaikai's hideout, capturing the defeated Shie Hassaikai's members, take over Overhaul's experiments as well as carrying the injured Heroes to the ambulances. At 9:15 AM, the Pro Heroes rescue operation to save Eri has been completed.

Kai Chisaki is subsequently arrested. Due to his injuries, he is transported to a villain's hospital, but the police escort is attacked by the League of Villains. Despite the intervention of a hero, the efforts of Tomura, Dabi, Mr. Compress, and Spinner prevail and Tomura proceeds to kick the immobilized and tied up Kai out of the crashed police van.

Unfazed, Kai asks if Tomura is planning on killing him. However, Tomura denies this and says that he has a better plan in mind for Chisaki which he will despise. Tomura admits that he despises Chisaki for his arrogance as Atsuhiro appears and then compresses part of Chisaki's left arm, agreeing with the statement.

Tomura checks the contents of the container but sees that there are two boxes, wondering which one has the finished product but sees that it does not matter. Chisaki demands that Tomura give the container back. Tomura touches Chisaki's right forearm and begins disintegrating him, noting the irony in Chisaki's plans to erase Quirks when he himself uses a Quirk to achieve that plan. Chisaki becomes worried as his arm is destroyed, as Tomura notes that all of Chisaki will disintegrate if they don't cut that arm off, so he takes out a knife which he uses to cut off Chisaki's arm.

Tomura then sadistically taunts the now helpless and powerless Kai at how he wanted others to lose their Quirks but now he ironically himself is one of them. Tomura gloats at having obtained Chisaki's life's work and laughs at Chisaki for being unable to use his Quirk, telling him that all he can do now is look on in envy for the rest of his life. Kai is left stunned at this before coming to realize he can no longer bring his boss out of his coma and screams in anguish at this turn of events, as he watches the League of Villains make off with his creation, with Tomura declaring that they will be the next rulers of the underworld.

A news report later details the attack on the police escort and Kai is subsequently apprehended and while detained his arms are medically treated and ends up imprisoned in Tartarus, though the extent of his injuries, aside from his arms, is unknown.

Paranormal Liberation War Arc
During the aftermath of the Paranormal Liberation War, All For One, using Tomura's body as his vessel, sends his army of Near High-End Nomu to invade Tartarus to retrieve his original body and free the prisoners located there, including Overhaul.

When the prison breakout occurred, Lady Nagant opens Kai's cell (as he couldn't open it without his hands), setting him free during the riot. He then rushes out of his cell, frantically calling out for his "Boss" while escaping. When All for One met Lady Nagant, he noticed Kai with her, with the latter insisting that she is using Kai for her plans. Sure enough, as the two were getting dressed to leave, Kai told Lady Nagant he knew who her target is, citing him as the one who ruined his plans. This caused her to bring him with her so he could identify him, with him asking for her to bring him to the Boss, as he no longer cared about anything else.

Dark Hero Arc
He is later seen standing on top of a building alongside Lady Nagant, watching as she manifests a gun from her body, firing a bullet from the gun with a microphone that allows her to speak to Deku, ordering him to come with her if he wants to keep his limbs. Kai muttered that he wanted to go to his boss, so Lady Nagant promised to visit him once she completes her mission while telling him to hide as Deku is approaching. Lady Nagant aimed her rifle at Kai while he yells if she has completed her mission because he held up to his end of their deal, so they can just go to the boss. This causes Deku to notice his presence. Lady Nagant shoots a bullet at him, though she adjusts the trajectory so that it wouldn't hit him. Izuku uses Faux 100 Percent and pushes Kai out of the bullet's way, then telling Kai he will talk to him later.

Shortly after Lady Nagant is defeated, Kai is apprehended by Endeavor and brought to Izuku and Hawks's positions. Kai shouts to the unconscious Lady Nagant that she has yet to complete their deal and demands to see the Boss, saying that he wants to apologize to him. Izuku asks Kai if he remembers everything he did to Eri, causing him to remember that he could restore the Boss with her power. Izuku understands Kai's remorse for the Boss, and states that if he is willing to direct those same feelings to Eri, he will fulfill his promise in Lady Nagant's place.