#//so many words just to say 'F1n's ability could tear down all of Qu1rk-Society if ever it was harvested'

[truly-quirkless]

This actually sprang up from an RP a hot minute back.

Fin's Quirk- Quirkless- could technically be distilled from their blood and given to others, much like Eri's ability. It's essentially the same if it's distilled into a bullet- shoot someone, they lose their ability to manifest Quirks.

But unlike Eri's Rewind Bullet, Fin's wouldn't have an 'antidote' that could also be taken from their blood. On top of that, Quirkless would be passed down through generations, effectively removing any Quirk of any future generation made by its victims. Quirkless- as a power- is built solely upon eternal suppression. Where Aizawa's Erasure suppresses (or rather, erases) a Quirk's capabilities for a time (every time he blinks), Quirkless in the blood stream has no limit. Once it's fully embedded, that's it. Unless Eri rewinds the victim, that person- and all their future progeny- will never manifest Quirks outside of Quirkless. They may have others, but those will be suppressed by Quirkless, itself.

Fin's Quirk, 'Quirkless', is a response to the Quirk Singularity Theory.- If Quirks start to overload those who have them, then Quirkless makes it so that overload never happens via eternal suppression.

And, of course, Fin can temporarily apply this to anyone within a ten foot radius for up to five minutes. That's because their version of Quirkless- their personal version- has evolved to suppress Quirks around them, which was done in a split-second panic when Fin thought they were about to lose Yagi (Eri's Rescue/The Yakuza Raid Arc- really those two have a habit of saving each other Istg). If they had any offspring (they can't, Fin's infertile)- their offspring would not be able to suppress Quirks around them, just anything in themself that's not Quirkless.

And finally-

The only reason All for One didn't take Quirkless from Fin for his own personal use was because Quirkless doesn't have an off switch. It can be pulled back/grow exhausted and retreat- but only to the extent of one's own body. If he'd taken Quirkless, every Quirk All for One had would've become suppressed forever. Quirkless' base power- to suppress- was amplified by the other Quirks in Fin's family tree and mutated. One would have to have about the same power as a black hole in order to actually suppress Quirkless, itself. Technically, AfO could achieve that, but he'd have to give up on a lot of concentration, which isn't worth it since he's trying to get OfA.

I write this on the assumption that All for One has a Quirk somewhere in his arsenal that basically tells him the details of any Quirk he encounters/attempts to absorb or something- possibly 'Quirk Data' or something like that. And when he realized what Fin's was after trying to overload their body with Quirks, he just threw them into the air. They were still so weak from the kidnapping that he figured they'd die- but thankfully, the U. A. kids were able to save Fin from splatting against the ground, and got them out of the fight.

Overall, Fin doesn't exactly realize most of this- nor do any of their allies- but Quirkless could be used to completely erase Quirks off the face of the planet- forever. It could be used to stop All for One, like Eri's bullet. It is the destruction of a Quirk-filled society.

Thanks to this kind of ability, Fin probably would have gotten dragged into the final war arc whether they liked it or not- because a Quirk like theirs would dismantle the world All for One wanted to build (or, raise it up, since he could keep Fin alive and just continually harvest Quirkless from them to use on all of the rabble who refused to follow him/weren't worth killing or worth keeping their Quirks- but for that to happen he'd have to keep Toshinori alive, which would probably be fine since the dude would essentially be in a living Hell if that future came to fruition).