Rewind

Rewind (巻き戻す) is the Quirk used by Eri. It is the result of an extremely rare mutation, and bears no resemblance to any Quirk on either side of her family's lineage.

Description
Rewind allows Eri the ability to bring a living being's body to a previous state.

When Rewind is activated, Eri's horn grows and glows brightly. Those that are in contact or in the vicinity of Eri at the moment of the activation start having their bodies rewound to previous physical states, undoing or bringing back injuries or modifications from the present or the past. She is capable of reverting people back to a point before they even existed, effectively erasing them from existence. Kai Chisaki even deduced that this Quirk can rewind genetics, dubbing it a terrifying power.

Rewind, as confirmed by Neito Monoma, requires accumulation of a yet unknown element in order to be activated, otherwise the Quirk will remain dormant, with Eri's horn remaining small and stubby.

Usage
Because of her young age and inexperience, as well as lack of information about Rewind, Eri is unable to properly control her ability, leading to the erasing of her father in one occasion. Its nature as a Quirk that affects only living beings also restricts its potential for training due to the high risks involved. Because of this, Shota Aizawa is one of the few people that can stop Eri if she ends up losing control.

Through several tests and experiments, the Shie Hassaikai was able to develop a drug that could target a person's Quirk Factor, rewinding it to a Quirkless state. The drug consists mostly of Eri's blood and cells, which contain the properties of her Quirk.