The Shie Hassaikai is one of many Yakuza groups that ran the criminal underworld in Japan. A man known only as "The Boss" originally lead this organization and raised Kai Chisaki into it. The Boss believed that gangsters would only follow a boss with a code and morals. His generation fell from prominence during the rise of All Might and Heroes. The Yakuza were pushed into the shadows and lived under constant surveillance.
Kai grew up feeling he needed to restore the Yakuza to their former glory because he owes the boss a great debt. He also strongly believed that Quirks were a plague on the world and people infected with "Hero Syndrome" ruined the Boss's life. Kai got more heavily involved in dark dealings despite the Boss's misgivings.
Eventually, the Boss introduced Kai to his daughter's child. A little girl was left to him after her Quirk made her father disappear into nothing. The mother believed this girl was cursed. Kai looked into it and discovered that Eri's power was the frightening ability to Rewind. He developed a plan to use Eri's Quirk to develop a Quirk-Destroying Drug that would destroy Quirks and restore the Yakuza to power.
Kai would use his own Quirk to overhaul Eri's body and experiment with her DNA and infuse her blood into a weapon that erased Quirks. Kai believed he could cull the Heroes by developing "Quirk-Killer Bullets" and distributing throughout Japan. This would bring guns back to prominence as a legitimate threat to people and then the Yakuza could sell a cure too after the plan gets in motion. By controlling an entirely new market for Quirks, Kai could reinstate the Yakuza as the biggest superpower in Japan. He would achieve his goal of getting rid of Quirks and returning the world to the way it was before the Quirks changed everything.
However, the Boss refused to consent to Kai's inhuman plan and worried about Kai's sociopathic mental state. He tried to get Kai to understand the Yakuza's code of ethics and told him to leave if he refused to embrace them. Disappointed but ever determined to achieve his goal and honor the Yakuza, Kai secretly put the boss into a comatose state. He took on the name Overhaul and became the young Yakuza boss of the Shie Hassaikai. 
Overhaul first shared his goals with Hari Kurono and Joi Irinaka, the only two people he grows to trust. He recruited anyone he deemed useful into the fold, even those who were considered trash, so long as they followed orders and use their Quirks for his ambitions. People followed him because he gave them a purpose. Kai believed himself to be enlightened and his pawns to be expendable bullets for his assault on society.
The gangsters who followed the Boss only followed Overhaul for him. They believed Kai was involved in his sickness and disliked how the organization and its members became associated with villains. The small Yakuza group became a dangerous villain organization created with the intention of destroying Quirks and returning the Yakuza to power. Following the fall of All For One, Overhaul deems it the perfect time to become the next king of darkness.
The Yakuza are still a small organization on the brink of extinction and do not have ample resources for Overhaul to carry out his plan. The Eight Precepts need to collect funds for their operation. They commit a series of unresolved crimes where they escape with the stolen money, including one incident where Overhaul attacks Team Reservoir Dogs. The Yakuza keep their hands clean by using Overhaul's Quirk to heal the people they attack.
The Shie Hassaikai also supplies Trigger (a Quirk enhancing drug) to dealers around Japan as a means for income. Overhaul is eventually contacted by Twice and recruited to the League of Villains. Overhaul agrees to meet with the League so he can recruit them to work under him instead. This enrages Big Sis Magne and she attacks Overhaul.
Kai is forced to defend himself and uses his Quirk to brutally murder Magne. Mr. Compress tries to capture Overhaul, but his reinforcements on standby shoot him with one of the Quirk-erasing bullets. At this stage, the bullets are incomplete and Quirks eventually return. This opening gave Overhaul enough time to blow Mr. Compress's arm clean off his body.
Overhaul tries to defuse the situation and tells Tomura Shigaraki to contact him once things have cooled off. The Yakuza continue accumulating funds and experimenting on Eri, but one day she gets away from her minder. Overhaul chases her out into the city streets where she finds solace with heroes in training Deku and Lemillion. They recognize that she's been abused and Overhaul is forced to show his killer instinct.
Eri recognizes his bloodlust and runs back to him so he doesn't attack. He brings her back to headquarters, kills the minder who lost her, and reminds Eri that she's the key to the plan. Mimic interrupts, telling Overhaul he has a call from Tomura Shigaraki with his answer.
A black communion between the Hassaikai and the League of Villains commences a few days later. Tomura meets with Kai after being lead through their underground base. Tomura sits down with his feet up and immediately announces that if the League should join forces with Yakuza, it will not be under the Hassaikai, instead, they will be equals.
Tomura's second condition is that Overhaul shares the details of his plan because the League won't be involved in worthless schemes. Tomura reaches into his jacket and insinuates he has an idea, but Chrono and Mimic threaten him. Tomura is mostly unaffected and demands compensation for the death of Magne. Overhaul calls them off and Tomura reveals the Quirk-Killer Bullet that was used on Mr. Compress and asks if the plan has anything to do with it. 
Kai invites Tomura to play a game of Shogi but he refuses. Kai explains the fun of Shogi is using the opponent's pieces and asks that Tomura lend him Himiko Toga, Twice, or Kurogiri because he doesn't want them moving about. Tomura initially refuses, but Kai insists that the League return the favor since Kai did reveal his entire plan. 
Despite their misgivings toward Overhaul, Twice and Himiko become temporary Yakuza. Overhaul apologizes for killing Magne and asks that the new recruit share their abilities upfront. Both Himiko and Twice explain their Quirks and Shin Nemoto uses his own Quirk to make sure they don't plan on betraying the Yakuza. Overhaul asks that as wanted criminals, both the temps stay on base for the time being. 
Sir Nighteye's Agency launches a full-scale investigation of the Shie Hassaikai. They recruit several Pro Heroes and heroes in training for an intelligence meeting that ends with the decision to launch an assault on the Yakuza to rescue Eri. Eri's new minder encounters Sir Nighteye at the store while purchasing a toy and this gives Sir Nighteye the location of the Yakuza base. The Police and a large team of Heroes assemble at the Shie Hassaikai Compound a few days later. Overhaul notices they've finally come for him. 
Kai visits the Boss on his sickbed and apologizes for the upcoming battles by telling his elder it will get a little loud inside the base. The fighting begins at 8:30 AM that day. Rikiya Katsukame launches a preemptive strike against the heroes. Ryukyu's Squad takes him on while the others storm the base. Minor Yakuza thugs try to stop their entry but minor heroes deal with them as the main heroes get inside the base.
Overhaul and Chronostasis take Eri and begin fleeing the base through the underground passageways. Kai always figured the Police would catch on, just not this quickly. He refuses to allow himself to be captured and releases the Eight Bullets to buy him enough time to escape.  Mimic enhances his Quirk with the Trigger drug and merges himself with the cavern walls of the unground labyrinth. He controls the area while Kai puts up fake walls, both to cover his escape.
Sir Nighteye leads the Heroes down into the underground labyrinth and they're quickly met by Mimic's resistance. He stalls everyone except for Lemillion, who uses his Quirk to phase through the passageway. Mimic drops the Heroes into a room where Setsuno, Hojo, and Tabe ambush them. Suneater takes the triad on alone while the others escape. 
Despite their considerable abilities, Suneater defeats Setsuno, Hojo, and Tabe with his incredible talent, much to Mimic's surprise. Since the three of them couldn't accomplish anything, Mimic tries to take out the heroes one by one. Mimic tries to send Eraser Head to another ambush, but ends up sending Fat Gum and Red Riot to face Rappa and Tengai. 
The Heroes come out victorious in the battle of spears and shields. Rappa discloses Overhaul's plan to reinstate the Yakuza and reveals it's in its final stages. Chrono questions if this is the end of the Hassaikai but Overhaul claims it will live so long as he exists. Overhaul believes this incident will serve as the perfect launching pad for distributing the Quirk-Killer Bullets. He orders Twice and Toga into action in order to make sure his plan comes to fruition.
Mimic pushes himself against the Heroes, forcing his Quirk to its limit with attempts to crush the heroes as they traverse through the labyrinth. Eventually, he receives reinforcements and separates the heroes. Himiko ambushes Rock Lock and incapacitates him. She impersonates him and nearly injures Deku before Eraser Head stops her. She stabs him as well before retreating. Meanwhile, Twice and his clone of Rappa ambush Sir Nighteye and the others.
Sir Nighteye repels Twice and forces him to retreat. Twice and Toga recall Tomura's confidence and them and they decide that Mimic and the Yakuza are worthless. They both taunt Mimic, calling the Yakuza and its Boss useless. Irate, Irinaka warps the entire cave in an attempt to crush everyone. . Twice and Toga betray Irinaka and reveal his location to the heroes so they can capture him, effectively using the heroes to get back at Overhaul the Yakuza. Before Kai can make a clean escape, Lemillion catches up to him. 
Overhaul remembers Lemillion from their first encounter and sicks Nemoto and Sakaki on him. Sakaki attacks Mirio's body with his Quirk while Nemoto tries to discourage Mirio about his failure to secure Eri the first time. Nemoto believes himself superior to the other Eight Bullets and tells Sakaki that he'll join Overhaul as he accomplishes his ambitions. Lemillion refuses to falter and suddenly defeats both Nemoto and Sakaki with a Super Move. He follows up by quickly saving Eri from Chronostasis' clutches. 
Annoyed, Overhaul orders Eri back to his side and reminds her she's a curse meant to destroy others. Lemillion berates Kai for talking to Eri in such a cruel manner, but Overhaul responds by activating his deadly Quirk. Chrono takes aim with a gun loaded with the finished product and threatens to erase Lemillion's Quirk for good. A large battle ensues and even while protecting Eri, Lemillion proves to a true hero. Lemillion is stronger than even Kai and bests both of them in battle.
Mirio tells Kai that he's lost, but Overhaul claims he left that name. Nemoto craws back to his master's side and Overhaul arms him with the Quirk-Killer Bullets. Desperate to defeat Lemillion, Kai orders Nemoto to shoot. Nemoto takes aim at Eri and Lemillion sacrifices his Quirk to protect her from the pain. Even so, Mirio Togata protects Eri from the villains for another third of the battle without his Quirk. Overhaul commends Mirio's incredible talent and prepares to kill him when suddenly Deku and the rest of Nighteye's Squad storm the battlefield. 
Deku and Eraser Head tag team Overhaul and beat him down. He becomes increasingly irritated by their interference and orders Chrono to wake up from Lemillion's assault. Chrono takes Eraser Head out of the battle and Overhaul's Quirk returns to him. Greatly aggravated, Overhaul disassembles Nemoto's body as well as his own. Refusing to allow the heroes to win, Overhaul sacrifices Nemoto and his own humanity to take on a powerful, monstrous new form. 
Overhaul heals his wounds and admits he's never been pushed so far in battle. He reveals to everyone that Lemillion's lost his Quirk and then attacks. Nighteye engages the villain and orders Deku to take Mirio and Eri to safety. Sir Nighteye uses his Quirk to see a future where Overhaul wins and escapes. Pushed beyond his limit, Sir Nighteye succumbs to Overhaul's strength and is mortally wounded. Deku rejoins the battle and Overhaul argues that the Heroes will all die for opposing him but Deku claims he will change the future. 
Chrono captures Eraser Head and binds him using his Quirk. Deku and Overhaul engage in a fierce battle. In spite of pushing himself to the limit, Deku is nearly defeated by Overhaul's overwhelming new power. Overhaul uses Nemoto's Confession Quirk to lead Eri back to him. He claims Eri doesn't need to be saved and prepares to finish the Heroes off when suddenly Ryukyu Squad crashes through the roof with Rikiya in tow. 
Overhaul takes Eri and tries to escape through the new hole in the roof. However, Eri is inspired by Deku and Lemillion's bravery and decides to act so that she can save everyone. She uses her Rewind Quirk to separate Nemoto from Overhaul and jumps into Deku's arms. Overhaul desperately tries to get her back, but Deku accidentally launches himself outside the cave. Overhaul had been trying to avoid touching Eri because of her Quirk. Desperate, Overhaul tells Eri he belongs to her and decides to fuse with Rikiya this time.
Overhaul enters his final form and chases Deku to the surface. He tells Izuku that Eri's Quirk will rewind him into nothing if he continues to hold her. Overhaul claims Eri is cursed and he's the only one who can control her. Deku refuses and uses Eri's rewind to control his Quirk at 100% of its power.
Overhaul explains that Eri's power can destroy the current world and return it to the way it was. He believes that only he understands that she is the key to returning the world to order. Deku and Overhaul clash in the climactic final battle. With power equal to All Might's, Deku clearly outmatches Overhaul even in his final form. He proves way too fast to allow Overhaul to heal himself and the evil crime boss is finally defeated. 
Suneater and the Police rescue Eraser Head and arrest Chronostasis. Overhaul makes one last stand but Eri's power explodes and rewinds him to his regular form. Eraser Head stops Eri and the mission is finally complete. All eleven injured villains from the Hassaikai are arrested and transported toward a villain hospital via the highway. However, before they reach their destination, the convoy is attacked by the League of Villains. 
Tomura and his gang dispatch the police and confront Overhaul. They steal the Quirk-Killer Bullets and cut both of Kai's hands off as revenge for looking down on them and killing Magne. Tomura marches off with Kai's life's work as he watches helplessly while the Shie Hassaikai crumbles for good. .
The fall of the Yakuza has earned the Hassaikai a reputation for being a small-time gang. Their resources and wealth are very limited, forcing Overhaul to recruit any scum he can get his hands on. The gang's foundation is made from civilized thugs who are loyal to the original boss. These thugs are weaker than the average Pro Hero due to their less than impressive Quirks. The gang's arsenal includes handguns, knives, and katanas among other weapons and were only used by Overhaul as cannon fodder.
Under Overhaul's leadership, the Hassaikai maintain their small-time status as a ruse by mostly operating under the radar and covering up their crimes, pretending to uphold traditional Yakuza values. Mimic directs the organization while Chrono acts as Overhaul's direct aid. Both of Overhaul's assistants possess unique Quirks, but it's their minds he makes the most use of.
Overhaul recruits muscle from the underworld of society, regardless of their past. He established Eight Expendable Bullets that he would use to ensure the security of his master plan. Each of the Eight Bullets possesses considerable abilities and are very capable fighters. They are strategically paired into teams to maximize their strengths. For example, Kendo Rappa's Strongarm makes him a perfect spear in battle while Kenji Tengai's Quirk serves as the perfect shield. With the Eight Bullets and Overhaul's two assistants, the Hassaika became a very strong villain group, unlike the Yakuza thugs that were forced to work under them.
The Young Boss of the Shie Hassaikai is one of the fiercest villains in the series. Overhaul prefers to allow his underlings to do the dirty work, but his impressive physical tools, analytical skills, and deadly Quirk make him more than a match for most heroes.
Battles and Events
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