Demons Among Slayers? (Male Demon Reader x Kimetsu No Yaiba)

Evolution.

After Y/N took both Tanjiro and Nezuko to their room, he said goodbye and left to the seclusion that is his own bedroom. The only problem was...he couldn't sleep.

Y/N: "You've got to be fucking kidding me..."

Y/N groans in frustration as he sits up from the rather comfortable bed, knowing exactly why this was the ONE time he had trouble sleeping.

The little speech he gave to Mitsuri earlier was really messing with him. He couldn't sleep because he was afraid that when that time came, he'd fail. He wouldn't be able to prove himself. In the heat of the moment, he thought, "If an Upper Moon walked through those gates, I'd fight them." but once he gave it some thought, he was hit with the unavoidable question...How?

How could he hope to beat another Upper Moon? If Akaza or Kokushibo walked through the doors to his room right now, they'd kill him. He'd stand no chance. It bothered him.

Tired from moping around and trying to force himself to sleep, Y/N hops off the bed and grabs his sword before practically stomping out of his room, only to be greeted by Muichiro, whom he almost ran into.

Y/N: "Oh. Sorry, didn't see you there-"

Muichiro: "You should watch where you're going."

Y/N: "Yeah, I know, that's why I apologized."

Muichiro: "You're already a walking hazard. Don't make it even worse by inconveniencing people."

Muichiro then walks off without a care in the world, leaving Y/N to contemplate on what the fuck just happened.

Y/N: (The hell is wrong with that kid??)

Y/N, after deciding that Muichiro wasn't worth stressing over, made his was into the nearby forest, trying to find a somewhat secluded area that also had enough open space for him to use flame breathing without setting the forest on fire.
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It took a bit of searching, but Y/N managed to find a nice open clearing where he could let loose and not have to worry about forest fires or not having enough space.

But now, he was stuck with anew dilemma. He was alone. Sure, he could practice his breathing, but he wouldn't gain much meaningful experience that way. But he also couldn't wake someone up to train with him.

Y/N: "I should have thought about this before coming here. Should I just wait until tomorrow?"

Y/N sighs as he turns around to walk away, only to be met by a voice.

"You're just going to walk away?"

Y/N, startled by the sudden voice, draws his sword as he turns to face it, only to be met with nothing.

Y/N: "I know that voice....Shibo?"

"Are you okay with being weak?"

Y/N: "What...where is that...no..he's not here. Is that my head?"

"Don't walk away. Face me. Like a warrior."

Y/N tries to forces on where he'd heard these words before, and why he's hearing them now. It brought him back to one of the last times he'd seen Kokushibo.
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Y/N: "But I'm not a warrior! I can't even keep up with you while you're holding back!"

Kokushibo: "My holding back does not matter. It is called training for a reason. I am not trying to kill you."

Y/N: "But you're not even trying! And you're still winning!"

Kokushibo: ".....You think I'm not trying? What gave you such an assumption?"