Eri becomes a key player in My Hero Academia when she accidentally runs into Izuku Midoriya and Mirio Togata while the boys are in the middle of their internship at the Sir Nighteye Agency. Unfortunately, although Deku notices clear signs of abuse on her, he has no choice but to relinquish her into the custody of Overhaul, a man whom they do not trust and later grow to despise.

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For the most part, Eri is quiet and shy, in large part due to her horrible experiences at the hands of her so-called "father," but she truly shows the extent of her capability in Midoriya's final battle with Overhaul. Thankfully, the heroes successfully protect her, and she eventually begins to let go of her past traumas and truly starts being the child she is supposed to be. (Please note that there are a few manga spoilers involved.)

Shie Hassaikai Leader's Grandchild

Shie Hassaikai Boss

The group known as Shie Hassaikai plays an important role in the lawless underbelly of Musutafu city, at one point controlled by an indomitable leader referred to as The Boss.

This is the man that "adopts" Kai into the fold, only to be betrayed later when he turns into the villain known as Overhaul. The Boss is allegedly Eri's grandfather, but he sadly allows Kai to take care of her, resulting in the latter using her Quirk to produce his "drug."

The True Nature Of Her Quirk

My Hero Academia (Eri Joins Forces With Deku)

It's a bit confusing when introduced at first, but it becomes obvious that the little girl is burdened with one of the most powerful—and, therefore, one of the most baffling—Quirks ever shown in the series.

Rewind doesn't work on non-living entities, but anything with a heartbeat is subject to its overwhelming powers. It can, much like Orihime Inoue's Phenomena Rejection technique in Bleach, revert any occurrence, even de-age people into non-existence.

The Physical Aspects Of Her Quirk

Eri and Deku

The interesting thing about Rewind is its association with the spiral horn poking out the upper right side of her temple. Apparently, the magnitude of this anatomical structure relates to the overall level of her Quirk; for instance, the bigger it is, the more she is able to employ Rewind.

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Further, the aggregative nature of her power implies that the horn only has as much influence as the amount of "energy" stored within itself. In short, it cannot be released at random.

Is Rejected By Her Mother

Eri looking terrified in My Hero Academia

Eri's Quirk seems to have been triggered at a surprisingly young age, as it is mentioned that she inadvertently causes her own dad to vanish into thin air with Rewind.

This event shocks her mother into giving up on her permanently, leaving her immensely distraught and at the mercy of her "adoptive father," Overhaul. Due to these miserable experiences, Eri tends to be timid and cloistered, quite the antithesis of what a child her age ought to be like.

The Source Of Her Name

Eri

Eri, or 壊理, consists of the words "broken" and "truth," suggesting a rather dark overtone to her character. One of the meanings of 壊理 could be "annihilation," which refers to the catastrophic consequences of Rewind.

Further, this could be taken to signify the incomprehensible quality of her Quirk, considering that it's an entirely different mutant variety than any others in the current timeline.

Kept In Medical Quarantine

My Hero Academia Eri Quarantine

After the fight against Overhaul in which she supports Deku's victory, Eri's horn shrinks drastically, and she slips into a long period of unconsciousness. Deku mentions that her Quirk is far too unwieldy for her to truly regulate without any accidents, which convinces the authorities to place her under constant supervision, as they are afraid of Rewind's potential.

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In fact, Eraserhead is given this task, being the single hero with the capacity to nullify the triggering of her Quirk, although he wouldn't be able to alter any completed outcomes.

Eri's Blood Works Wonders

Overhaul

Eri's Quirk aside, her blood has the capacity to directly negate the hitherto indestructible "Quirk factor," that is to say, the genetic component of an individual responsible for their superpowers.

Unfortunately, this is gravely misused by Overhaul, who analyzes Eri's blood so as to produce highly specific projectiles that can effectively destroy another's ability permanently. As horrifying as it sounds, Eri undergoes years of torment due to Kai's inhumane experiments on her body.

Practices Rewind On Small Creatures

Eri using Rewind on Lizards

In order to gain some semblance of mastery over Rewind, Eri starts practicing her Quirk on tiny animals, such as lizards, insects, and so on. This way, she is able to release only tiny quantities of power each time, which drastically reduces the chances of anything backfiring.

Not much about this training regimen is explained, but the manga shows her detaching lizard tails and presumably restoring them back over and over again.

Brings Back Lemillion

Eri restoring Lemillion's Permeation while Eraser Head looks on - MHA

One of the newer groups of villains is the Paranormal Liberation Front originating with the help of Shigaraki Tomura, the supposed "successor" for All For One. When the heroes decide to mount an operation against them, Mirio Togata is eager to join, but, at this point, his Permeation remains destroyed.

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He goes to Eri and humbly requests that she rewind his body back until his Quirk returns to him. She happily agrees, and gently holds his face with her luminous palms; the process works perfectly.

Eri and Christmas

Eri as Santa

In the UA dorms, the gang decides to throw a Christmas bash to take a break from their grueling schedule, to which Eri is invited. She arrives wearing an adorable Santa costume, cut down to her size and transformed into a skirt.

However, being totally unaccustomed to how festivals work, she mixes Christmas up with Halloween and Easter. Eri offers 1-A candy eggs as presents, announcing her entry with "Trick or treat," which just makes everyone more cheery than ever.

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