The following contains spoilers for My Hero Academia Chapter 355, "Extras," by Kohei Horikoshi, Caleb Cook and John Hunt, available now in English from Viz Media.
The battle against All For One has taken an interesting turn. Besides Endeavor and Hawks, Class 1-A's own Kyōka Jirō and Fumikage Tokoyami have joined the fight. Tokoyami almost makes sense because of his connection to Hawks. However, Jirō's presence on this particular battlefield seems out of place.
As far as anyone is concerned, be they Pro Heroes or the audience, Kyōka Jirō has no business in this battle. Neither her Quirk nor her character seems like they would be necessary components to this fight. AFO even calls her out on these points and refers to her as a "deluded extra," which she kind of is. Regardless, she's determined to contribute whatever she can to this monumentally important fight. Here's what she adds to the field starting from Chapter 355, "Extras."
Jirō's efforts to be relevant in this important battle are best considered based on her past contributions to the story. As a fighter, she's always been kind of average among her peers; this is to say nothing of how she measures up to the heavy hitters in her class like Midoriya and Todoroki. Her biggest moment in the story prior to this was the "Hero Too" performance during the "UA School Festival" arc; granted, this was an amazing performance, but it didn't really say anything about her battle prowess. Her going up against AFO might be a little extreme, but it would certainly prove what she's really made of.
Unfortunately, Jirō's own mobility in this scenario is severely limited. Since this is an aerial battle and she can't fly, she has to rely on Tokoyami and his Dark Shadow to remain airborne. She also needs Hawks' feathers to help her out of the way of AFO's attacks; even if this battle were on the ground, she simply doesn't have the reaction time to dodge on her own. In these regards, she's more of a hindrance than a help.
She mainly tried to contribute to the fight with her new attack, Heartbeat Surround Legato. Unfortunately, AFO, for how powerful he was, remained virtually unfazed. Not only that, but he was gearing up for a powerful attack as he was being blasted. It's clear that in a one-on-one fight, Jirō would lose every time.
Luckily for Jirō, she's not alone. Besides Tokoyami and Hawks, another disturbance came in the form of AFO's stolen Quirks; their vestiges rebelled against him and impeded his movement. Since this had never happened before, not even under New Order's influence, AFO surmised that they were "spurned by the weakness of [these] pushovers." If he's right, then Jirō and the others challenging someone clearly out of their league actually serves as yet another contribution to the fight.
While Heartbeat Surround Legato didn't do much on its own, it was helpful in providing support for Hawks' attack. By running his Feather Blades along Jirō's soundwaves, he was able to add vibration to them. This Soundwave Vibration-Enhanced Flight Feather Blade was strong enough to break through a weak spot in AFO's life support system and shatter it. This will definitely have a profound effect on the battle in the long run.
However the rest of this fight goes, Jirō can leave it feeling proud of what she helped accomplish. She stood up to one of the most dangerous villains to ever live and effectively helped to mortally wound him. Her willingness to fight against the odds may have also caused AFO's Quirks to rebel against him; whatever caused this, it will definitely hurt him as the battle progresses. Even if she gets taken out in the very next chapter, she's already done far more than any "extra" could have hoped to do in this situation.