Eri is a small girl with bluish, off-white hair, messy and unkempt, which is parted in the middle of her forehead, almost reaching down to her waist. She has very wide, innocent-looking eyes, which are bright red in color. Poking out from the right side of her forehead is a small, brown horn, which grows larger when her Quirk is activated.
In her first appearance, she's wearing a plain, short-sleeved dress, which is a dirty, pale tan color. Bandages are wrapped around both her arms and legs, stopping at her wrists and ankles, underneath which she is shown to have numerous scars.
After she's taken in by U.A., Eri's outfit is changed to a white dress shirt with a frilled collar and a plain red pinafore with two large, golden buttons on either side of her chest. Under this, she wears gray tights and a large set of tan boots, and a small dark red messenger bag with the strap sitting over her right shoulder, and a little floral pattern decorating one of its sides. Her hair also looks to be better cared for, appearing straighter and neater than it was during her time with the Shie Hassaikai.
Because her mother disowned her after she accidentally made her father disappear with her Quirk and the experiments she was subjected to by Kai Chisaki, Eri became a fearful and secluded girl, without anyone showing affection to her in her life. Apparently, Izuku Midoriya's touch was the first time she ever felt kindness, which overwhelmed her. When murderous intents are directed at her, she does not scream, nor cry and simply remains silent, because she knows she has no choice but to accept whatever is to happen to her.
Eri is, however, willing to let herself go with Kai in order to protect those that help her. She feels guilt over other people being hurt in order to rescue her.
Unfortunately, after her rescue, Eri still psychologically suffered from Kai's influence, as she was unable to smile and had no experience of being happy. However, this changed, thanks to the events of the U.A. School Festival, and Eri finally smiled for the first time.
Rewind (巻き戻す Maki Modosu?): Eri's Quirk allows her to reverse a living individual's body back to a previous state, allowing her to make her target physically younger, heal injuries, and undo bodily modifications. She has even shown the ability to rewind someone's body to a point before they existed.
The source of her Quirk's power is located in the horn on her head, and the size of the horn dictates the strength of her power. It is also an accumulation-type Quirk, which means the power it emits has to build up over a period of time before becoming usable.
As a result of her Quirk mutating her body, Eri's blood is able to attack an individual's Quirk and stop its activation by reversing the target's evolution before the point of the Quirk Phenomenon.
It was suggested by Shota Aizawa that her Quirk can even rewind the bodies of those who have lost their Quirks and restore them. However, Shota said that the Quirk only works on people, not on things, which means that it's not something that can be easily trained. This also means Eri cannot use her Quirk on Time as it is not a living thing.