My Hero Academia imagines a far-future setting where superpowered Quirks are commonplace, and these Quirks have redefined what is and isn't possible for humanity. In the hands of pro heroes, Quirks can be used to save lives and prevent crime, but Quirks also have their downsides, such as the Quirk singularity theory. Eri is an example of that.
The young Eri was rescued in the "Shie Hassaikai" story arc, and her Rewind Quirk allows her to regress a person to an earlier state. She once used that ability to heal Izuku during the battle against Overhaul, and in theory, Eri could heal anyone, like a new and improved Recovery Girl. Perhaps she could restore Mirio's Permeation Quirk as well, but there are risks involved.
Can Eri Restore Mirio's Powers With Her Quirk?
Eri's Rewind Quirk is one of My Hero Academia's strongest, and when used, it rewinds a person to an earlier state, hence its name. This can drastically alter the target, as Eri can heal practically any wound by reversing the person to a state before they sustained that injury, which is what she did to help Izuku use One For All's true power and defeat Overhaul. In addition, Eri's Quirk can be weaponized to reverse a person to before their Quirk manifested, effectively erasing it. That is why Overhaul used Eri to create Quirk-erasing bullets, and that's how Overhaul wiped out Mirio's Permeation Quirk. But while Eri can heal people's injuries with her Quirk, rewinding the effects of her own Quirk is a different matter.
Mirio's loss of his Quirk is no ordinary injury. It was the effect of Eri's Quirk in weapon form, and the Rewind Quirk can't so easily undo its own effects. Rewind works best on external factors, while it can't so easily rewind itself, even if Eri tries. In short, Rewind only works in one direction, and two negatives don't make a positive where Mirio's Quirk loss is concerned. Eri's Quirk was already used to reverse his body to before it had the Permeation Quirk, and reversing that effect won't reverse things again and make Mirio's Quirk go forwards. Eri can't reverse a reverse, and if she tried, she would likely lose control of her abilities and make things worse. She might even reverse Mirio to a time before he was born, making him vanish entirely.
Why Mirio Won't Ask Eri To Help Him
Both Mirio and All Might have their reasons to ask Eri to help them, but Eri can't so easily aid either of them for similar reasons. To begin with, Eri probably cannot rewind a rewind with her Quirk, and even if that weren't an issue, there are other problems. Eri is still just a child, and she bears a tricky and powerful Quirk without the benefit of formal training. She's liable to lose control of her Rewind Quirk and do a lot more harm than good with it, and everyone knows it. It's too risky to ask Eri to use her Quirk under any circumstances -- for now, she is focusing on rehabilitation and catching up on her education. Her guardians, Mirio and Aizawa Shota, are focusing on that, and they want to distance Eri from her Quirk for the foreseeable future.
Even if that weren't an issue, there is still the psychological distress Eri is going through. She associates her Quirk and its effects with years of mistreatment and exploitation at the hands of Overhaul and the mobsters, and no doubt this makes Eri frightened of her Quirk, regardless of whether she can safely control it. That, combined with her age, means she may experience serious distress if she uses her Quirk on anyone, and that's not what her new guardians want for her. The heroes are desperate to use anything and everything they can to fight the villains, but they won't act like villains themselves by pressing a frightened, mentally scarred child into service as an unwilling support hero. That would defeat the very purpose of being a hero. Mirio and All Might must somehow recover on their own.
