The latest battle between the heroes and villains resulted in many casualties (and even deaths) on both sides in the My Hero Academia manga -- not to mention the escape of Shigaraki once more. But one good thing did come out of it: The triumphant return of Lemillion. Mirio had lost his Quirk in his battle with Overhaul, and although he had faith that he could still become a great hero without it, we knew that one of U.A.'s goals was to hone Eri's skills enough that she could help him regain his Quirk.
And she did just that: Not only did Eri bring back Mirio's Quirk but she also brought some hope back for the heroes. Whatsmore, if Eri can master her Quirk, she could be a real force to be reckoned with in future battles, especially as a source of healing.
It's unlikely that Eri will ever use her Quirk for combative reasons so it makes more sense for her to be in a supporting role as a healer. Unlike Recovery Girl, who can speed up the body's natural healing process, Eri's Quirk, Rewind, isn't a healing power in the conventional sense, but it does allow people to return to a previous state. This lets her heal injuries and reverse any physical changes that the body undergoes.
Her Quirk is also the type that has to accumulate power. We saw an example of her potential as a healer when Midoriya was battling Overhaul. There was absolutely no way he could've beaten him without dying if he hadn't had Eri. Midoriya had so much faith in Eri's abilities, believing that her Quirk was truly a blessing, that he felt safe enough to go into Full Cowling 100%. As a result, he suffered no injuries while Eri was using his Quirk and could go all out. But, in his haste to bring down Overhaul, he didn't realize that Eri's hold on her power was still fragile. By the time he did, it was almost too late -- until Aizawa stepped in.
Eri's Quirk has the potential to return people to their previous state but it's extremely volatile: She once accidentally 'Rewound' her father to the point where he didn't exist. We also saw how dangerously powerful she could be when Chisaki weaponized her Quirk so he could create a Quirk-less society. So, a counter-argument to her being merely a support hero can be made, in that her destructive potential makes Eri a formidable foe in battle. But there are several reasons why this wouldn't work.
For one thing, her control over her Quirk right now is unpredictable, and though she healed Mirio and he regained his Quirk, it's unclear just how in-control she is or if it's replicable at this point in time. She had been experimenting on lizards and insects for two months before she tested it on Mirio, after all. Secondly, because of her time with Chisaki and the nature of her father's death, she always thought her Quirk was a curse; that her very existence was cursed.
Seeing Mirio and Midoriya risk their lives to protect her made her more scared than anything, leaving her feeling burdened with guilt and self-hatred. Her time at U.A., -- having friends and having Aizawa as her guardian/teacher -- has helped with her deep-seated psychological trauma but there's a large possibility that if she were asked to use her Quirk to fight, it would be extremely triggering. It's doubtful that even if she trains herself to the point where she has full control over Rewind, she won't ever be truly free from that fear.
Using her Quirk for healing purposes will help with some of her trauma. Instead of using Rewind to hurt people, she can reverse people's state to the point at which they were never hurt in the first place. When she used Rewind on Mirio, she was crying because she was so happy that she could save the hero who saved her. Thanks to Midoriya's encouragement and with Lemillion back in action, Eri is finally able to see that her Quirk isn't a curse but a blessing, putting her on-track to maybe becoming Recovery Girl 2.0 in the future.
