It's fucking freezing.

You shiver and cough as you use your arms to haul yourself upwards, having lost all sensation in your legs from earlier. Your two pistols lie uselessly in the grass a few feet away.

Out of the corner of your eye, you can see Tanjiro and Inosuke cowering in fear at the battle being waged in front of you.

Just... a little bit more. Grab your pistols and treat them.
   
The ice-cold pre-dawn air chills you to the bone as you drag yourself forward by your fingernails to grab your pistols. You managed to grab both of them before crawling over to Tanjiro.

"Tanjiro, over here," you wheezed as you flipped open your medical kit to reveal an array of bandages, medicines, syringes, and knives. "I'll treat you."

When the conductor stabbed him earlier, there was some sort of poison coating his dagger. I don't know what it was and I don't think Tanjiro knows what it was either. It's best not to use anything specific lest it make it worse. I'll have to run a blood test on him when we get back for further treatment.

You flipped him over so he was in the recovery position, upper leg pulled slightly forward and lying on his side. Digging through your medical kit, you found what you were looking for.

Activated charcoal.

"Tanjiro, I'm going to give you a shot that will slow down the effects of the poison. This activated charcoal will bind to the toxins, which prevents the body from absorbing it."

You pour some into an empty syringe, adding some water and mixing it thoroughly before injecting it into his arm. He winces at the pain before you bandage the puncture site, wrapping a length of bandages around and around his arm.

"You just got poisoned not even ten minutes ago, so this will be especially effective. Inosuke, I leave him to you."

After patching up both of their wounds, you scan the area, looking for any more injured people when a loud boom from the fight in front of you startles you so badly you almost drop your medical kit.

"Don't mind the weaklings, Kyojuro! Especially that (h/c) woman. She's especially weak, I can tell!"

You gasp sharply, being reminded once again of your shortcomings. Tanjiro and Inosuke glance at you nervously, gauging your reaction.

There is no response from Kyojuro. For a moment, you think that the sounds of battle have drowned out his response, when his voice rings out across the field, filled with rage.

"Don't you dare insult her like that! She's far stronger than you give her credit for!

"Breath of Flames, Fifth Style: Flame Tiger!"

The clash of flame against snow leaves you temporarily blinded. You cough at the dust that is generated, along with Inosuke and Tanjiro.

You catch Tanjiro staring straight ahead, horror in his eyes. With growing dread in your stomach, you follow his line of sight to see Kyojuro battered and beaten, panting heavily. Blood drips from his forehead and mouth, staining the front part of his uniform a sticky red. His left eye is sealed shut with blood dripping out of it as well. From the looks of it, his left eye is smashed, he has multiple broken ribs, and his organs have been ruptured, leading to a dangerous case of internal bleeding. 

"Kyo..." you whisper, not daring to believe the sight in front of you.

"Rengoku-san..."

There was no opening for you to jump in. You were nowhere near their skill level and if you did, you'd be dead in less than a tenth of a second.

How? How? How do I help him? If I try to shoot Akaza, he'll just dodge my bullets and kill me like I'm a bug...
   
You could see Inosuke and Tanjiro had the same thought. Tanjiro attempted to hoist himself up but found that he couldn't even stand, as he could put no strength in his limbs.

There is a cough from Kyojuro. He steadies himself, flame haori billowing out behind him.

"I will fulfill my duties! I will not let anybody die here!"

He shoots a glance back at you over his shoulder as if to reassure you that everything will be alright. You clench your fist.

At that moment, you can't help but think that he looks especially majestic; like the rising sun.

"What a wonderful fighting spirit- you have so many wounds, yet your drive, your fortitude, and your stance have no openings. You really should become a demon, Kyojuro! Let's continue to fight for all eternity!"

"Breath of Flames, Ninth Form: Purgatory!"

"Technique Deployment! Destructive Kill: Obliteration Style!"

They move so fast that you can't keep up. One moment he's in his battle stance and the next, he's moving across the field in nothing more than a blur of flames so hot you can see the air ripple and warp around them.

There is a loud explosion, and then nothing. A billowing tower of dust surrounds and clouds the horizon, leaving no hint as to what just happened.

"I can't see..." whispered Tanjiro next to you.

The dust begins to clear, revealing the heads of Akaza and Kyojuro.

"Ah..."

A horrified sound, something between a gasp and a shout, leaves your lips upon seeing Akaza's arm sticking out of Kyojuro's torso.

"Rengoku-san!"

"Kyojuro!"

He lets out a pained cough, even more blood dribbling down his chin.

"It's too late for you, Kyojuro!" cackles Akaza in glee. "I purposefully wounded my arm ahead of time. At this moment, my blood is surging through your body. You'll become a demon, Kyojuro! You'll be able to reach supreme territory and fight for all of eternity!"

You can't breathe. It feels as if the air has turned to solid ice around you. The world is spinning and reality no longer feels like reality anymore.

Kyojuro is silent in front of you momentarily. You see his hand twitch.

"(Y/N)-san, get down!"

You're pulled back by the collar of your haori as Kyojuro leaps towards you with inhuman speed. Out of pure luck and survival instinct, you block his incoming attack with your two pistols, which collide with his fangs. You're forced onto your back and you stare up at him with fear.

Two horns are protruding from his forehead and the sclera of his left eye is now dyed a pitch-black. Cat-like slit pupils bore into yours as he growls at you, teeth scraping against the Nichirin steel of your pistols.

"Rengoku-san! What're you doing?! She's your girlfriend!"

You let out a gasp of pain as his nails dig into your shoulder, ripping the fabric and scraping your skin.

Shit, he knows what human blood smells like now! I have to get him off and kill him before it's too late!

You attempt to kick him off, only to find that your legs are pinned underneath his. Even if they weren't, you'd still have a difficult time as you were pretty sure you had multiple broken bones in your legs.

At times like this, you cursed your physical weakness.

"Stop it, Kyojuro! Please, it hurts!"
He doesn't seem to process your words, only continuing to growl at you. You wince and place your finger on the trigger.

Just pull the trigger, (Y/N), and it'll all be over. This is your responsibility as a member of the Demon Slayer Corps; to exterminate and destroy all demons on the face of this planet.

But you can't. You can't pull the trigger no matter how much you force yourself to.

"Please, stop this, Kyojuro! Don't you recognize me? I'm your girlfriend, (Y/N) (L/N), the girl you said you loved more than life itself!"

He continues growling at you, but you swear you see a flicker of recognition in his eyes. Tanjiro, Inosuke, and Zenitsu are doing their best to hold off Akaza behind you despite their injuries.

"You said that we'd get married one day and have lots of kids. You said you wanted at least three, remember? You joked that Senjuro would be overjoyed to be an uncle, right? You know, your precious little brother?

"Right, Kyojuro?

"Please, answer me!"

A/N: Hello everyone! This is going to be my first kny x reader work and I do hope you enjoy it!

(Y/N) will be referred to with she/her pronouns.

There will be manga and movie spoilers. After the Mugen Train arc, just know that each chapter will have manga spoilers.