Izuku Midoriya vs. Kai Chisaki

Izuku Midoriya vs. Kai Chisaki is the final battle fought between U.A. Student Izuku Midoriya against the Hassaikai Boss Overhaul during the climax of the Shie Hassaikai Raid.

Eri uses her Quirk to help Deku unleash One For All at 100% to combat Overhaul's final form after he fuses with Rikiya Katsukame.

Prologue
After being tricked by Himiko Toga, Ryukyu Squad goes through several floors to defeat Rikiya Katsukame, crashing into the underground where Izuku and Overhaul are fighting. Everyone is confused by this unexpected introduction, but Overhaul decides to take advantage of the situation, and tries to escape with Eri through the opening in the cave.

Deku goes after Overhaul in an attempt to rescue Eri. Overhaul creates stone pillars with his Quirk that rapidly raise him to the surface with Eri. Eri notices Lemillion's cape flying through the air and is reminded of everyone's desire to save her. Seeds of hope sewn by Lemillion have sprouted within Eri to grant her the determination to save those around her. Eri inadvertently activates her Quirk, undoing the fusion between Overhaul and Nemoto, and leaps into Deku's arms, who promises not to let go of her this time.

Overhaul looks down at them from a stone pillar of his own creation and demands Eri return to him. He destroys his own footing and descends at his enemy before overhauling the mass into a series of spikes that he sends at Deku.

Deku knows he can't dodge Overhaul in mid-air, but he refuses to let go of Eri. He recalls Mirio asking what kind of hero Izuku wanted to become. Deku wishes to be a hero that can save everyone and winds up a counter-attack using his Shoot Style.

Suddenly, Deku creates a shockwave that propels him and Eri to the surface while still in the middle of his swing. He surmises that he used too much of One For All and the wind pressure created from his kick sent them flying, in the same manner All Might used New Hampshire Smash to move through the air.

Aggravated, Overhaul calls out to Eri. He tells her that he needs her to fulfill his ambitions and claims no one seems to understand his plan is necessary to change the world. He stumbles over to Rikiya Katsukame's unconscious body and fuses with him. Deku lands on the street above ground and is surprised he didn't break his body despite going over his limit. However, as soon as Eri lets go of him, Deku feels the repercussions. Overhaul announces that Eri can't control her power and rises out of the cavern in a monstrous kaiju-like form.

Overhaul explains that Eri's Quirk allows her to rewind people, either physically or genetically. He claims that Deku will disappear if he continues to cling to her because all her power does is destroy. Overhaul tells Eri that she's cursed and says the only way to stop her is with his disassembly. Deku refuses to return Eri. He puts her on his back and wraps Mirio's cape around him to bind them together.

Deku surmises that Eri's Quirk stopped him from feeling the pain from overusing One For All. He calls her Quirk kind, which brings Eri to tears. Deku states that Eri can heal him with blinding speed, but if he can accumulate injuries at an even higher rate then he can avoid rewinding too far. Deku asks that Eri lend him her power as he unleashes One For All: Full Cowl at 100%!

Battle
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.

Within the blink of an eye, Deku uses his blinding speed to bypasses all of the Overhaul's attacks and kicks the massive beast high into the air. Eri's power is getting more powerful as she's going all out to help her hero. She doesn't have full control over her Quirk and is going full throttle just like Deku did when he first got One For All. Infuriated that no one understands that he's trying to change the world fo the better, Overhaul goes right back at the young hero and transforms his giant body into a typhoon of mass.

Deku instantly flies through Overhaul's attack and break it apart completely. He knows there's a time lapse between Overhaul is able to heal himself after taking a hit. With his newfound speed, Deku propels himself back at his enemy and exploits Overhaul's weakness with a powerful barrage of punches at using 100% of One For All.

As Sir Nighteye watches on, he realizes that Deku is changing the destined future that he foresaw. Deku believes that if he cannot save a little girl right in front of him, then he may never become a hero who can save everyone. Deku delivers an emphatic final blow to strike Overhaul's mask off with a fist that opens the path to the future.

Deku grabs one of Overhaul's giant limbs and throws the entire monster into the ground. Overhaul lays defeated and Eri loses control of her Quirk. It's getting stronger even though the battle is over. Chisaki remembers getting into a fight when he was younger because someone said the Yakuza were accursed villains. The boss told a young Chisaki 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 stone hand that tries to crush Deku. Eri's power explodes into a wave that rewinds Overhaul all the way back to his original form. Uravity quickly secures his hands and restrains him.

Aftermath
Eri's power is exploding and Deku is struggling to keep One For All's power flowing through his body. Eri is afraid she'll kill him and calls herself cursed for having this Quirk. Froppy helps Eraser Head look at them both so he can cancel out their Quirks. He saves the young champions and they immediately fall unconscious, victorious in their mission.

Later on, the ambulance arrives and Deku talks to Sir Nighteye. Grievously injured, Sir tells his pupil that he managed to change the future he had foreseen. Izuku asks Sir to hang on so he can reunite with All Might. Sir Nighteye thanks Deku for making the mission a success.