Shie Hassaikai

The organization began as one Yakuza family among many holding pieces of the Japanese underworld. A man remembered only as The Boss ran it and raised Kai Chisaki inside it, convinced that gangsters would follow a leader only if that leader kept a code. All Might's arrival and the professionalization of heroics ended that era, driving the Yakuza into the shadows and keeping them under permanent watch.

Owing The Boss a debt he felt he could never repay, Kai decided the way to settle it was restoring the Yakuza to what they had been. He also came to see Quirks as a plague, blaming people infected with what he called Hero Syndrome for ruining the old man's life.

The turning point arrived with an introduction: Kai met the old man's granddaughter, a girl left in his care once her own power erased her father from existence. Kai investigated, found that Eri could Rewind, and built a plan around her: rework her body, experiment on her DNA, and refine her blood into a Quirk-Destroying Drug whose bullets would cull heroes and open a market the Yakuza could corner by selling the cure alongside the poison. The Boss refused, alarmed by the cruelty and by Kai's mental state, and told him to embrace the family's ethics or leave. Kai chose a third option, quietly putting the old man into a coma and installing himself as the young boss under the name Overhaul.

Overhaul shared his real goals with only Hari Kurono and Joi Irinaka, and recruited anyone useful regardless of how disposable he privately considered them, so long as they obeyed. People followed because he handed them a purpose; he regarded himself as enlightened and his followers as ammunition to be fired at society. The old guard never accepted him, suspecting his hand in the Boss's illness. With All For One fallen, he judged the moment right to install himself as the underworld's next king.

Money was the immediate obstacle, so the group ran a string of unsolved robberies, one of which involved attacking Team Reservoir Dogs, and kept its hands clean by healing the victims afterward. Supplying Trigger to dealers nationwide brought in more. Twice approached him for the League of Villains, and Overhaul took the meeting intending to absorb them instead, which enraged Magne. He murdered her, and when Mr. Compress moved on him, reinforcements fired an unfinished Quirk-erasing round and Overhaul blew the man's arm off.

The experiments ran on until Eri escaped her minder and found Deku and Lemillion in the streets, who read the abuse immediately, though she returned to Kai to keep him from attacking them. A later meeting with Tomura Shigaraki settled terms: the League demanded equal footing and compensation for Magne, and Overhaul disclosed his plan and borrowed Toga and Twice. Sir Nighteye meanwhile built a full investigation, and a chance encounter with Eri's minder at a store handed him the base's location.

Police and heroes converged on the compound days later. Overhaul and Chronostasis fled underground with Eri, releasing the Eight Bullets to buy time, while Mimic dosed himself with Trigger and reshaped the labyrinth walls to cover the escape. The tunnels became a sequence of ambushes: Suneater beat Setsuno, Hojo, and Tabe alone; Fat Gum and Red Riot took the spear-and-shield pairing of Rappa and Tengai; Toga and Twice, deciding the Yakuza were worthless, goaded Mimic into warping the cave and then gave up his position. Lemillion caught Overhaul and outfought both villains until Nemoto fired the finished bullets at Eri and Mirio took the shot himself, sacrificing Permeation to spare her.

Cornered, Overhaul disassembled Nemoto and fused with him into a monstrous form, mortally wounding Sir Nighteye and nearly finishing Deku before Ryukyu's squad crashed through the roof. Inspired by the two students, Eri used Rewind to tear Nemoto free. Overhaul fused with Rikiya instead and pursued them to the surface, where Deku, channeling Eri's power to hold One For All at full output, overwhelmed him. All eleven wounded villains were arrested, but the League ambushed the convoy, stole the bullets, and cut off both of Kai's hands as payment for Magne and for his contempt.

Overhaul ranks among the deadliest villains the series produced, preferring to let subordinates handle the dirty work while his physique, analytical mind, and lethal Quirk made him more than most heroes could manage. Chronostasis served as his direct aide and Mimic directed the organization; both had unusual Quirks, but it was their thinking Overhaul valued. The original Boss stayed comatose through the entire affair.

The Eight Bullets were the muscle, recruited from the underworld without regard for their pasts and treated as expendable insurance. Overhaul paired them to cover weaknesses, most memorably setting Kendo Rappa's Strongarm as a spear behind Hekiji Tengai's Barrier as a shield. Shin Nemoto, Rikiya Katsukame, Toya Setsuno, Yu Hojo, Soramitsu Tabe, and Deidoro Sakaki completed the unit, and each one inverts a Buddhist precept, running from Rappa's bloodlust against the prohibition on killing to Sakaki's Sloshed against the prohibition on intoxicants.

Below them sat the ordinary thugs, still loyal to the old Boss, weaker than the average professional, armed with handguns, knives, and katanas, and used purely as cannon fodder. Their loyalty made the small-time reputation useful cover for something far more dangerous. Every member was arrested, and the family was disbanded for good.