Official Quirk Explanation (This is important to understanding what's going to happen when she fights in the final round)
Because a lot of people are still a tad confused (I don't blame you it's not a very straightforward kind of quirk) I'm going to try and explain Y/n's quirk, the best I can and please don't hesitate to ask questions, I read every single comment so--
Y/N has a third nervous system. The human body originally has two- central and periphery. One is the brain and spinal cord (central) and the other is the nerves that branch off. Y/N's third nervous system is an extension of those nerves that work by sensing things outside her body. For example, when it rains, you can feel the raindrops when they fall on your skin. Y/n can feel the increase in specific elements (water / H2O) before the drops even touch her body. That's her quirks first ability: sensing elemental states and quantities at the atomic level. Second, after she senses these conditions, she can alter them. Rain is just two hydrogen atoms and an oxygen atom interacting. If she wanted to, Y/N could use her quirk to pull apart the two hydrogens and oxygen by willing it. That's her quirk's second ability: manipulating atomic bonds.
As for limitations: Y/n can't create or destroy any elements, that's actually impossible (even though most quirks are lol). So she can't just create oxygen when she needs it. She can, however, displace oxygen, meaning manipulate pressure so that the oxygen floats upward or downward, making the exchange of oxygen between her cells more efficient (that's why she doesn't run out of breath, her body does this on its own).
Important facts:
The reason her ears ring is because of the frequency. When atomic bonds are broken it releases energy. Since Y/N does all she can to suppress her quirk, she's containing that energy which builds up like pressure. It causes tinnitus. This is also symbolic of her denial. She doesn't want her quirk to exist, therefore she chooses to not hear it, which causes her pain.
Nervous systems function, for the most part, involuntarily-- meaning you can't control the actions of those systems by will like you'd move your hand. Y/n's quirk is similar. If she is in physical distress, her third nervous system works the same way any cell does. It'll protect the body without her choosing.
***
No one truly understands the size of a person till they have to carry one.
But you did. Every bone, every ounce of blood, you could hear and feel in others since you could remember. You knew the exact weight of every single one of those cells, not from experience, but rather from concept. Still, you weren't prepared for carrying a fourth of Bakugo's weight on your shoulders.
Probably because the weight of his expectations for you was even greater...
There wasn't an official announcement or anything of the sort.
A single swing of Midnight's whip, a ring of a bell, and suddenly it was a free for all.
Alright.
Thinking time.
10 teams. Only 4 make it out.
The majority of them weren't powerful enough to even consider going for your headbands.
However, Sero and Mina joined. And though with you and Kirishima standing at the front you made formidable allies, your leader was obnoxious enough to attract too much attention.
He wanted to go for Mydoriya.
Fair enough.
But you needed an opportunity.
And his team wasn't exactly defenseless.
Uruaka touched one of you and it was over.
Tokoyami's dark shadow was already leading as the front horse, blocking and deflecting any teams that went for their headbands.
Add a crafty little support course techie and they had all they needed.
Bakugo didn't waste a single second.
When Izuku's team took to the sky, so did he.
Controlled explosions, using his nitroglycerin, adding some heat, and bam, the pressure raised him to right where he wanted to be.
"Don't think for a second you're safe. I've got you!"
"Tokoyami!"
Obeying his leader's order, Dark Shadow encased his fellow team members, shielding them from Bakugo's offenses.
"Sero," You called, watching Bakugo start falling back down. "Catch him."
"On it."
Once his tape wrapped around Bakugo's arms and pulled him back to you four, it was a waiting game.
You had around 600 points, but 10 million? Bakugo wasn't going to give up without it.
His want for this win radiated off his body.
It was almost humbling.
Made you wonder for a moment if you could've used your quirk to keep him up there just a few seconds longer for the opportunity to strike again.
But using your quirk wasn't an option being surrounded by so many.
"Nice catch." You winked back at your friend, Kirishima and Bakugo having a small yelling match over what to do next.
That all stopped when another team you hadn't even thought of noticing entered the equation.
Looking over your shoulder, you only caught the final moment of Bakugo's headbands being swiped from his possession.
600 went to zero all too quickly.
"Your class is too small minded." The other team's leader, blonde haired with a smug expression and careless eyes spoke, twirling the stolen headbands around his finger. "Think bigger."
"He got us!" Mina worried, Kirishima quickly joining in.
"Crap." He bit. "We forgot about 1-B."
Everyone on your team's dynamic quickly shifted from strategy to panic.
Well, everyone but Bakugo.
"What did you say!?" He yelled, flexing his hand in a choking motion. "Give that back! I'll kill you!"
Good.
He was still confident.
That was enough to put you all back in the right headspace.
You turned your attention back to the offensive.
Monoma. That was his name right?
You remembered from the line-up Endeavor made you memorize.
His quirk wasn't listed or known, which meant it wasn't showy just like yours.
Okay. That was handable.
He was a rider. That meant some sense of leadership or at least charisma. From the way he spoke, there was a lot of arrogance to mix in there as well.
Okay. That was exploitable.
"Midnight said the obstacle course was just the first game. And we figured they wouldn't cut that many of us right off the bat. Would they?" He spoke, a smirk widening his face as his horses turned to face your team.
Okay. He liked to talk. That was exploitable too.
You didn't have to win this, but those points were too valuable to lose.
If you were going to stay with Shoto and help him, you needed to be in the top four.
You weren't making it there with nothing.
Monoma. You thought. If you would just get a little closer and shut the hell up so I could knock you to the ground with some wind pressure, that would be lovely.
"Assuming they'd keep at least 40 contestants for the next event, all we had to do was make sure we stayed within that group as we ran." He wasn't close enough yet, but his words made you stop thinking for a second.
"Shit." You realized. "They observed your quirks in the race."
"Only a fool would obsess over winning the preliminary rounds wouldn't you agree?"
Kirishima stepped forward, "You planned this as a class?!"
The boy looked down at you both with an upturned chin, reaching up to wrap the headbands around his forehead.
"It wasn't totally unanimous," He said and you could feel the irritation in Bakugo's face. "but I'd say it's playing out well. It's better than chasing after a temporary front runner like a horse with a carrot waving in its face."
Your leader growled out a threat, planting his hands down on both you and Kiri's shoulders and squeezing.
Monoma just chuckled and continued, pleased he was able to arouse such a reaction from your resident hothead.
"Oh, while I have your attention, you're kind of famous aren't you?" You shivered, but it wasn't you he was speaking to. "For being attacked by that sludge villain? You'll have to tell me about it sometime. I mean c'mon, it must be strange to always find yourself in the role of the victim."
Got you. You thought.
Monoma messed up.
While he was wasting his time trying to antagonize Bakugo, you noticed his eyes flicking down to you every few seconds. And when Bakugo grabbed your shoulder and Kirishima used his hardening, he waited for your quirk to activate all the same.
But it never did.
"Kirishima!" you called, your friend turning his head to face you in response. "He doesn't know what my quirk is. If I can get within range, I can grab the headbands--
"Headset." Bakugo interrupted, his throat rumbling with anger in the way he called for your attention. "We have a change of plans."
Before Bakugo had a chance to take on his persona where he looked like the literal devil, red eyed with his body tense as a rock, one of Monoma's horses interjected.
"Monoma! Don't provoke him man, that's the kind of thing he would do."
"Yeah you're right. Guess that's not very heroic of me." The boy smirked, "Plus you know how these things work. Heroes are always getting hounded by villains desperate for some kind of revenge."
Your eyes widened at his audacity.
But they quickly thinned as your lips rose around your teeth in a snarl.
"Don't joke about that, you pompous ass!" You yelled, leaning forward as much as you could like a dog ready to attack someone who'd threatened him. Even Bakugo was a bit surprised at the sudden intensity, but he'd be lying if he said it didn't please him to see you so invested in kicking this guy down.
"Calm down guys." Kirishima urged, not sure why you and Bakugo suddenly seemed like the same person. "You've got to keep a level head or we'll never get our points back."
"Shut up!" Bakugo yelled, raising himself back up straight with the widest smirk you'd ever seen. "I've never been calmer."
Rolling up his sleeves and flexing his hands, readying them for his quirk, he looked down to you.
"Headset." He said, waiting for you to look back up at him. "Don't let me down."
Blinking up for a few seconds, you let yourself take in just how proud he looked.
Not of himself. Not of his skill.
Just of being able to fight for what he wanted.
Matching his expression, you answered wordless with a nod, before spinning back around.
"Kirishima." You said, feeling the weight of Bakugo's body leap off yours' and propel itself towards your opponents. "Use your quirk."
"Huh? Why?"
"So that I don't hurt you."
Confused, Kirishima stuttered over a few words.
But he saw the focus in your eyes.
There was no deterring that, so instead he just listened and hardened his body as he was told.
Keeping your eyes on Monoma, you waited for just the right moment as Bakugo flew past him.
"You think you're tough?!" He yelled, swinging his arm to aim an explosion at his rival. "Die!"
Just as the explosion went off, you let out a breath and let time slow around that singular blast from Katsuki's hand.
The carbon dioxide released from the nitroglycerin and the oxygen, it contained energy in its combustion.
It was just enough energy to redirect,
Outstretching your arm and swiping your wrist upward, you managed to control that small amount of power and cause an updraft.
An updraft one would feel in a rainstorm.
As the air blasted all around them, 1-B's team was left clumsy and confused.
It wasn't enough to knock them into the sky, but just enough to distract them.
Monoma barely clung to his teammates while they were barely able to stand on their feet.
Unfortunately for them, headbands had no feet to stand on nor hands to hold on.
Apply enough pressure and they move to the wind's demands.
As the four of you ran to catch Bakugo, you managed to grab those 600 points.
"Coward!" Bakugo yelled, not satisfied with Monoma still standing, but before he could deliver another explosion and finish him off, he was hit with a blast directly in the face.
Not just any blasts too, but one of his own.
As you ran past to catch your falling teammate, Monoma managed to reach down and touch Kirishima.
Not hurt him, or move him out of the way, or use him as balance.
Just touch him.
You furrowed your brows, Bakugo's weight falling back onto your shoulders, distracting you for one precious and needed second to understand just what Monoma's quirk was.
"Damn it!" Your leader yelled, regaining his footing and delivering a blow aimlessly towards his opponent.
This one was a tad too loud for comfort, and so close to your ears it made you wince.
But there was no time for that.
Because it didn't even seem to phase, the boy in front of you.
"You all got impressive powers.But I think you'll notice mine's better." He said, the smoke clearing around his body, revealing arms that looked too familiar blocking his face.
They were hardened.
Just like Kirishima.
"His quirk." You mumbled, "He can--
"He can copy other powers." Bakugo finished for you.
The way he said it was like it didn't even matter that this was an ability beyond useful for both attack and defense.
To him it was just another obstacle to knock down, something else to fuel the want to win, but this:
The ability to take a quirk from someone else?
It reminded you too much of someone you feared and a power you feared even more.
"So?" Monoma chuckled, trying to stall Bakugo long enough to steal the headbands back. "Even an idiot can figure it out."
Before he could get angry, you reached over your shoulder and tried to pull the shirt of Bakugo's uniform down.
He had to listen to you.
"I got our points back, we don't need him, please let's go." You said, Bakugo grabbing your hand and tearing it off him.
"Not until he's beaten."
The plea in your voice, he obviously didn't notice it, because the moment you turned around, the opposing team was far closer than you'd remembered.
You tried to flinch backwards the second you noticed Monoma's hand.
It almost knocked Bakugo off all four of you the way you threw yourself backwards.
But sadly, your reflexes weren't fast enough.
"Wait!" You yelled, Monoma's fingers just barely grazing your hair.
In the distance sounds of ice and collisions caught everyone's attention, but not yours.
Even though it was Shoto out there and Bakugo was surely focused on taking him and Deku down next, you couldn't help but keep your eyes on Monoma.
Your quirk sprang alive inside your body the moment he made contact with you.
It could sense your fear, sense the sheer panic in your veins.
"I don't know what your power is sweetie." Monoma cooed, flexing his fingers back and forth trying to get a feel for this new sensation. "Seems a bit abstract. But it feels..." He stopped, shivering and shutting his eyes. "powerful."
"Monoma!" You yelled, your voice higher, tone more desperate than before. "Don't use it, please, you don't know how to control it!"
But the boy only laughed.
Such was his mistake.
No. You thought. Because when you take a quirk that isn't yours, your body tries to adjust to it. And even if Monoma had no intent to use this power of yours, it'll still tear him apart.
Outreaching his arm towards Bakugo, Monoma let out a breath, ready to try his own.
You made a distressed sort of sound, not knowing what to do, except watch.
"Stop it! You'll hurt yourself!"
Kirishima, who was focused on Bakugo's attempts to redirect the team towards Deku and Todoroki, spun around to face you.
When he saw what you were looking at, he nearly stumbled backwards too.
Monoma's arm locked and suddenly he was starting to cringe.
A look of confusion on his face, the boy grabbed his own wrist and and started recoiling into himself.
He looked like he was in so much pain.
It's not his quirk. You thought. It's not my body. It doesn't know what to do with his.
Shaking from the neck down, Monoma started grabbing at his own skin, as if the quirk were a physical thing he could tear out.
All you could think of was how confused he must've been.
You'd been able to feel and drown out the sensations around you for years.
Even then, your quirk developed over time.
You could only imagine what it must've felt like to be hit head on with all that pure noise.
The noise of people's existence, the noise of the air, the earth below, the sensitivity to every single movement and every single chemical exchange between even a single atom.
Monoma grabbed his ears, pressing down with enough force to tear out of his hair, grimacing and squirming.
Finally, you gave in.
Unlinking your arm from Kirishimas, you started running towards Monoma.
"Y/n! Where are you going! Bakugo needs us!" Kirishima yelled, but you weren't listening.
If that stupid kid couldn't control your quirk from the inside, maybe you could from the outside.
Right before you reached him, Monoma started yelling in pain, banging himself in the head, his teammates letting him down on the ground, trying to understand how to help him.
"Get away from him!" You yelled, but you were too late.
Monoma crumbled and the moment his knees hit the ground, a shockwave thundered around him and through everyone within 10 meters.
The sound it made was deafening.
His friends were blown backwards, each of them still thankfully conscious, but one of them unable to stand up.
The whole entire crowd made a gasping sound, the other teams still thankfully too distracted to notice.
Aizawa, I could really use you right about now. You thought, still running at Monoma, unable to tend to his teammates.
Skidding down to your knees, you planted your hands around Monoma's ears and pushed till he was lying down.
"Stop. Stop. Stop. Stop." You said, feeling him fight this thing that was invading his body like a parasite.
"You son of a bitch," you ground out, speaking to your quirk directly. "For once in your life, shut up!"
And then, as if for the first time it finally listened, Monoma went limp and silent.