Eri is the granddaughter of the ex-Capo 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.
Izuku along with the heroes and police make their way to the underground labyrinth to arrest Kai and save Eri. 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. Understanding that Eri's Quirk reverts any pain and grievous damage back to normal at blinding speed, Izuku decides that he does not need to hold back since all major recoil and damage accumulated by his Quirk will be continuously reverted and healed thanks to Eri's Quirk, therefore not needing to worry about turning into nothingness. Izuku asks Eri if she will lend him her power and, going all out, activates One For All: Full Cowl - 100%.
Izuku has delivered 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 Expendables 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.
A couple of days after her hospitalization, Eri wakes up albeit still in a psychologically unstable state. Her horn, where her Quirk came from, had noticeably shrunk to the size of a lump.
At the hospital, Izuku and Mirio are finally able to see Eri. Before Izuku and Mirio enter Eri's room, one of the nurses ask 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.