Eri is the granddaughter of the boss of Criminal Yakuza, the Shie Hassaikai, through her mother. At a young age, she accidentally used her newly manifested Quirk on her father and it caused him to vanish. Her Quirk resembled neither of her parents' so her mother believed her existence to be cursed and gave the girl up to her father. Her grandfather then left her in the care of his protege, Kai Chisaki, so that he could test her to identify her Quirk as well be able to sympathize as it seemed as if they had similar Quirks. However, her Quirk was misdiagnosed and was in fact 'Rewind' and not 'Vanish'.
After her grandfather fell into a critical state of illness, Kai took over the organization and then began to experiment on Eri to turn her Quirk into a weapon. Through unknown means, he discovered that her blood had properties that suppress the bodies' ability to manifest 'Quirk Factor', inhibiting someone's Quirk. He manufactured them into bullets that are 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 Quirk-Destroying Drug that he believed could permanently destroy Quirks.
Eri was running down an alley away from Kai, mentally chanting she had enough of her treatment and wishing someone would save her. She bumps into Izuku Midoriya and Mirio Togata, and Kai catches up to them.
Kai apologizes to Izuku for his daughter's behavior because she gets carried away with her fun and games. 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 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. Kai politely asks Izuku not to impose his idea of normal on others' 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.
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.
Back at the Shie Hassaikai's hideout, the subordinate in charge of watching Eri tries to apologize for letting Eri out of his sight but Overhaul kills him on the spot for his failure, telling Chrono to clean up the mess. Overhaul takes Eri to his experimentation room. He tells her to stop being selfish because she's the crux of his operation. As Chrono removes Eri's bandages, Mimic tells Overhaul that he's getting call from Shigaraki from the League of Villains.
Days go by, and Kai ordered one of his subordinates to be the new caretaker of the girl, with the specific orders of become the kind of person Eri trusts. To achieve that, the subordinate treats the girl kindly and buys her a lot of toys, but none of that works. One, day, when he enters her room to check on her, he is surprised that Eri hasn't opened any of her toys. This frustrates him because he fears if he fails to gain Eri's confidence, Kai could kill him. Still, he continues with his facade of being a nice guy with Eri and pats Eri before leave. On her bed, Eri thinks about Izuku's warm embracing touch and how his hand felt different. She's never felt a hand so kind before, causing Eri to start tearing up.
After Kai's encounter with Izuku and Mirio, Sir Nighteye continued to investigate Shie Hassaikai, and with the information that was provided to him by other heroes, he managed to deduce that he is using Eri to produce his bullets. For them, it launches a joint operation between the police and various heroes to rescue Eri and dismantle the yakuza organization. Knowing the truth, and feeling guilty for not helping her, Izuku and Mirio are the ones who are most willing to save Eri.
For several days, Heroes from all across Japan continue to investigate all properties connected to the Shie Hassaikai, trying to find the place where they hold Eri. Sir Nighteye manages to find the place when he uses his Foresight in one of Kai's subordinates while buying a toy intended for little girls, discovering that he is Eri's caretaker, and thanks to that he knows she is in the Shie Hassaikai headquarters and how to get to her. With this vital information, he orders start raid immediately.
The next day at 8:30, the police force and the heroes surround the Shie Hassaikai headquarters. However, Overhaul had anticipated their arrival, and he and Chrono prepare the Eight Bullets to hold off the heroes until they can escape with Eri and their research. Izuku, along with Mirio, Sir Nighteye, Eraser Head and other heroes and the police make their way to the underground labyrinth to arrest Kai and save Eri. There are clashes between heroes and members of the Yakuza.
Using his Permeation Quirk, Mirio decides to get ahead of the rest of his teammates, eager to save Eri. Several minutes after advancing through the underground labyrinth alone, Mirio catches up with Overhaul and Chronostasis Overhaul recognizes Mirio Togata as one of the Heroes he encountered when Eri ran away.
Mirio Togata is able to catch up with Chronostasis and Overhaul. The villain is surprised that Mirio was able to get to them so easily; Mirio replies that he took a shortcut. Surprised, the villains believe Mirio shouldn't have been able to pursue them. Mirio claims 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. He claims that Eri doesn't wish to be rescued by him because he's already failed her when they first met and mocks Mirio for his futile heroics, to which Mirio replies that he came to rectify his mistake. Two of the Eight Bullets, Shin Nemoto and Deidoro Sakaki, appear to face Mirio and kill him, but the young hero manages to defeat them.
With no time to waste, Lemillion phases through the ground and appears before Overhaul and Chronostasis and attacks simultaneously, taking them by surprise. Overhaul varely dodge his punch, and Mirio manages to hit Chronostasis. Mirio avoids hitting Eri thanks to his Permeation and strikes Chronostasis down with his kick. Chronostasis is launched away 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.
Disgusted, Overhaul wipes dirt from his face and orders Eri to return to his side. Overhaul reminds her that when she acts selfishly that he's the one who ends up with his hands dirty. He claims Eri was born to destroy others and she pleads with Mirio to run without her. Before Eri can speak, Mirio tells her not to listen to him. 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, The villain reveals that was a lie and he has no children, as he places his hand on the ground and deconstructs it using his Quirk.
Overhaul quickly reassembles the floor into a bed of spikes that collide into each other in an attempt to crush Lemillion and Eri. Mirio phases his body while holding Eri up with his arms, protecting both of them, and is surprised at Kai's quick speed as well as his lack of regard for Eri's safety. Overhaul claims that if Eri is badly injured or even killed, he can just "repair" her with his Quirk and implies that he's done it many times before. This enrages Mirio even further, but he's forced to keep dodging to keep Eri out of danger.
Chronostasis has recovered from the previous attack and grabs a gun loaded with Quirk-disabling bullets. Chronostasis targets Mirio's arm that's cradling Eri while Overhaul erases his cover. Lemillion cloaks them both in his cape which throws off Chronostasis's aim. Chronostasis remarks that heroes wear capes just for show. They land and Mirio secretly phases through the ground and takes Chronostasis by surprise with an uppercut. 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 and punches him in the face, while proudly states that heroes wear capes to cloak tormented little girls in its comfort, and continues attacking Overhaul.
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 Overhaul lay another finger on Eri and promises to take them both down, declaring that he has them in checkmate.
However, a defeated Shin Nemoto manages to drag himself to the place where the fight was taking place. 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 and Kai orders him to open fire. Shin knows how valuable bullets are, so he cannot waste a single shot. He knows he can't shoot Mirio because he would avoid the bullet with his Quirk, but realizes that Mirio will do whatever it takes to protect Eri, so he aims his gun at her.
Refusing to allow her to be hurt ever again, Mirio immediately jumps towards Eri to protect her. Eri opens her eyes and sees a smiling Mirio who is about to be hit by the Quirk-destroying bullet. Mirio reaches Eri while Shin Nemoto fires the Quirk-destroying bullet. At the moment before the bullet hits, Mirio reaches her and gets her attention so she can see him giving her a reassuring smile that says everything is going to be okay. Mirio covers Eri as the Quirk destroying bullet is shot into his body which begins destroying Mirio's Permeation.
Overhaul mocks Mirio because everything he has worked so hard to cultivate has been reduced to naught because Eri. However, despite his Quirk has been destroyed, Mirio just keeps laying into Overhaul using nothing but the physical conditioning and foresight he trained to help him master his quirk in the first place. Mirio tells Kai that all his hard work is not in vain because he will still be Lemillion. He ends up several hits but he continues to fight and protect Eri for several minutes before getting badly wounded, buying just enough time for Izuku, Sir Nighteye and Eraser Head to smash their way into the room.
While Izuku and Shota deal with Overhaul, Sir Nighteye runs towards an injured Mirio, amazed that he has been fighting all by himself against the boss of the Shie Hassaikai and protecting Eri. Sir Nighteye grabs hold of Mirio and Eri. A proud Sir Nighteye praises and congratulates Mirio for everything that he has done.
Meanwhile, thanks to Chronostasis capturing Shota, Overhaul uses his Quirk to construct massive spikes from the ground, which stalls the heroes. He uses his quirk again to disassemble his own body and Nemoto's, then rebuild and merge into a single and monstrous being. By fusing with Nemoto, Overhaul recovered from his injuries and regained his stamina.
Overhaul rushes at Sir Nighteye, who's protecting Mirio and Eri, but Deku rushes into him and they struggle for a few minutes until Sir Nighteye rushes into the battle and orders Izuku to protect Mirio and Eri while he handles Kai. Izuku goes to Mirio and Eri's location with his enhanced speed, and after seeing that they are fine, he and opens a hole in the wall so they can escape. Eri apologizes because she considers that everything that is happening is her fault.
Sir Nighteye is defeated and is seriously injured. Kai goes past Sir Nighteye and chases after Eri. Seeing him, Izuku decides to stay to face him while an injured Mirio tries to get Eri away from the place. While Izuku fights a merged Kai, in the hallway, the damaged and tired Mirio Togata wills himself onward to grab Eri to safety. However, Mirio's body begins to give out due to wounds and fatigue. Before falling unconscious, Mirio tells Eri that she needs to hide and wait for the others to come and find her.
Back at the battle, Overhaul manipulates two pieces of debris and wedges them into Deku's arm and leg. He commends the aspiring hero for his persistence and compares it to Lemillion's. Wanting to end the confrontation as soon as possible, so a mouth appears in one of his hands, and shouts so Eri can hear him, asking her if she wanted all this. In the hallway, Eri stops in her tracks and remembers being disassembled and reassembled by Kai's hands. Eri runs back to Kai and replies to him that she never wanted any of this. A shocked Izuku tells Eri that she needs to stay with Mirio.
Kai, revealed to be using Shin Nemoto's Quirk, asks Eri if she thinks Izuku can overcome this situation by himself. Eri truthfully reveals that Izuku is not capable of doing so; Overhaul then asks her what she must do to resolve this situation, to which Eri replies that she must return to him and in exchange, Kai must fix everything back to normal. The villain simply replies that Eri has always tried to take the pain for others and because of that, she's unwanted. Izuku unlodges the spike from his leg and crushes it while loudly proclaiming that no one will die today and that he will save Eri. Then suddenly, Ryukyu's group crashes through the roof with Rikiya Katsukame in tow.
It turns out that they have been tricked by Himiko Toga, who had used her Quirk to transform into Izuku, and allow the League of Villains to have access to where Eri was. Without time to think about what was happening, Izuku runs to rescue Eri. However, Chisaki prevents Izuku from reaching her with a rock pillar, grabs her and creates a giant pillar that raises them both toward the newly formed hole in the roof. Overhaul creates a giant pillar and grabs Eri. Deku jumps and reaches out to Eri.
As she and Overhaul head for the surface, Eri sees Mirio Togata's cape caught by debris, and by instinct, she grabs the cape after remembering Mirio's kind words. Suddenly, Eri begins emitting strange energy which starts affecting Chisaki, and rewinds his body before he fused with Nemoto, separating them both. After so much time of having lost hope of escape, and resigned to being "cursed", Mirio's words shook Eri's heart to the core and couldn't bear to see others hurt for her sake; she was further shocked that they refused to give up on her, which caused her to start developing a new feeling, the feeling of needing to be saved, and leaps into Deku's arms.
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. Izuku isn't able to move well suspended in mid-air. However, Izuku vows to not let go of Eri no matter what and recalls Mirio asking what kind of hero Izuku wanted to become. Deku wishes to be a hero that always wins and saves everyone and winds up a counter-attack using his Shoot Style.
Filled with utter determination, Izuku unconsciously activates One For All 100% and attacks Chisaki with a kick. 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 power. Deku lands on the street above ground and is surprised that his legs are not broken and have in fact being healed, surprised that despite using One For All 100% his body along with all his injuries, has been healed. Izuku realizes that his healed body is thanks to Eri's Quirk ability. However, as soon as Eri lets go of him, Deku feels the repercussions.
At that moment, Overhaul appears in a monstrous kaiju-like form. In his desperation to recover Eri, he used his Quirk again to merge with Rikiya. Overhaul explains that Eri's Quirk allows her to rewind people either physically or genetically. He claims that she can not control her Quirk and 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.
Izuku vehemently refuses to hand back Eri and will hold onto her no matter what. Having experienced Eri's Quirk, Izuku sees that the moment his legs broke Eri reverted them back to normal before he could experience any pain and thinks that Eri's Quirk is a kind, gentle Quirk contrary to Kai's thoughts, which shocks Eri. 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%!
Fed up with Izuku's insolence, Overhaul attacks him with huge stone spikes, but Deku dodges the attack sends Overhaul flying high into the sky with one kick. Then, 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 knowing there's a time-lapse between Overhaul is able to heal himself, proceeds to speed blitz and exploits Overhaul's weakness with a powerful barrage of punches at using 100% of One For All, until he finally delivers a massive One For All: Full Cowl - 100% enhanced punch to Kai, which defeats him.
Kai's massive stone body falls onto the ground, with Kai himself grievously injured. As he lands on the ground, Izuku asks Eri is she is fine. Suddenly, Eri's Quirk starts going out of control. Kai wills himself back into consciousness and attempts to attack Izuku with his stone hand in a last ditch effort to get back Eri. However, Eri's Quirk spreads near Kai, which causes his fusion with Rikiya Katsukame to become undone, returning Kai back to normal. Ochaco Uraraka apprehends the defeated Kai.
Ryukyu returns to the surface with Tamaki Amajiki, Mirio, Shota, and Tsuyu Asui and asks for a report on the situation; Ochaco explains that they have called an ambulance for Sir Nighteye and that Izuku has defeated Kai but his power is spiraling out of control. In reality, it is Eri's Quirk that is causing Izuku immense pain. Tsuyu lifts the injured Shota so that he can see Izuku. Shota, thinking that he is erasing Izuku's Quirk, disables Eri's Quirk. Eri and Izuku fall unconscious as their Quirks are deactivated. Ryukyu orders to call for more ambulances and to confirm if there are anymore victims in the vicinity as well as search for any League of Villains members.
The Police Force and Heroes search the Hassaikai's underground hideout, capturing the defeated Eight Bullets as well as carrying the injured Heroes to the ambulances. Outside, Izuku hands over Eri to the paramedics, telling them that she has a fever ever since she lost consciousness.At 9:15 AM, the Pro Heroes rescue operation to save Eri has been completed.
Eri is sent to the hospital where she is admitted. She hasn't woken up since she was rescued and her fever is not going down. Eri has to be in quarantine because, based on Izuku's information about her, she is unable to control her dangerous Quirk. Therefore the most rational decision was to quarantine her for her sake as well as everybody else's, under Shota's surveillance since he is the only one who can stop Eri in case her Quirk gets out of control again.
At the beginning of October, a couple of days after her hospitalization, Eri wakes up albeit still in a psychologically unstable state. For this reason, Izuku and the others were still forbidden to see her face to face. Her horn, where her Quirk came from, had noticeably shrunk to the size of a lump when the fever went down.
At the hospital, Izuku and Mirio are finally able to see Eri. Before Izuku and Mirio enter Eri's room, one of the nurses asks if it is safe for them to be around Eri, to which the doctor states that Eri's energy level is safe and thus her Quirk will not spiral out of control. Izuku apologizes to Eri for being unable to see her earlier while Mirio gifts her a basket of fruits. Even though he saved her, Eri does not know Izuku's name, which surprises Izuku but he informs her anyway.
Eri apologizes to them for all the trouble for her sake; however, she is unaware that Sir Nighteye has passed away. Eri is guilty that Mirio lost his Quirk because of her, but Mirio tells Eri that she is not to blame and everyone is glad that she is safe, which comforts Eri. Eri tries to smile but is not able to do so. Izuku is aware that Eri has not recovered from her psychological abuse from Kai.
Suddenly, Izuku has an idea and asks Shota if Eri can come to the School Festival. Mirio explains to Eri that the festival that the U.A. hosts where there will be lots of events on display where people can enjoy themselves while tasting delicious food such as candied apples, which catches Eri's attention. As Shota phones Nezu to get his approval, Izuku asks Eri about his idea; Eri wants to get to know the people who saved her and agrees to go to the School Festival.
At the Festival, she and Mirio are enjoying the show which Class 1-A has set up and she is happy when she sees Izuku perform as the "ghost" of Kai leaves Eri, finally. She laughs for the first time and cheers with the audience.
Later on, Eri spoke to Izuku and states she enjoyed the performance but didn't get the candied apple. However, her disappointment turns to joy when Izuku gives her one as he stated he got the recipe and made one himself. Grateful and happy, Eri enjoys the gift.
Later, while the professors watch on television the fierce battle between Endeavor against High-End, Shota Aizawa asks Eri to go to her room, so she can not see the violent confrontation, and asks Thirteen to take care of her while he heads towards Shoto's location.
After the Joint Training Battle, Shota wants Neito to copy Eri's Rewind Quirk. Deku and Mirio are with her, since Shota did not want to leave Eri alone with Neito, in case his eccentric personality could affect her mental health negatively.
Neito attempts to copy Eri's Quirk. He grows the horn that is required for its usage, but says that he is incapable of using the Quirk itself because is an accumulation-type Quirk. Neito explains that he can copy the fundamental abilities of a Quirk, not something requiring stored-up energy as he cannot copy what has already been stored.
Neito apologizes for not being able to help more, but Shota thanks him for trying. Shota explains to Mirio and Izuku that as Eri doesn't understand how to properly use her power, he hoped that Neito's Copy Quirk would help her and make it easier for her to learn it as well.
Eri feels sad and apologizes for causing everyone trouble, saying everyone would be better off if she didn't have her ability, but Izuku reminds her that it was her power that saved him, stating that what she said is not true at all. He explains that what power is worth is all about how it's used, comparing it to how a knife is dangerous initially, but when used right can make all kinds of food, and saying that her power is an amazing one. Eri says that she'll try her best after all.
Eri makes an appearance alongside Shota at Class 1-A's Christmas party, dressed in a Santa Claus outfit. Likely due to her upbringing, Eri is unfamiliar with most holiday tradition and so she brings Easter Eggs rather than gifts as one usually does at Christmas. During the festivities, Izuku noticed that Eri's horn had grown some more.
While visiting his mother, Izuku reveals that Eri had hand written a letter to Izuku, thanking him for the invitation.