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

Evolution.

Y/N: "It's obvious! You're not taking this seriously at all, while I'm struggling to even get near you!"

Kokushibo: "...So you think because this is training, I have no reason to put forth any real effort, due to you being much weaker than I. Is that right?"

Y/N: "Well, yeah...that basically sums it up.."

Kokushibo: "So then...that would mean that you don't see any point in putting in any real effort either. Because you think I won't."

Y/N: "Well....ye-"

Kokushibo: "Okay. Then I will attack you with full intent to kill."

Y/N: "Wait-"

Kokushibo swings his sword at Y/N, who narrowly dodges the blade by jumping back and falling on his ass.

Kokushibo: "Do you realize what I just did? I just attacked with everything I had. Yet here you remain. Do you know why that is?"

Y/N: "You missed me on pur-"

Kokushibo: "No. It's because you'd seen it so many times. I always started by attacking in the same direction. You became accustomed to it. You expected it. Even at what I would consider close to my full speed, you still managed to dodge because you knew it was coming. I'm going to attack you again. Be ready."

Y/N quickly hops up in time to block the same attack, which was quickly followed by another. Y/N, reacting purely on his instinct, slides under it and attempts to cut Kokushibo, only for him to backstep, leading Y/N to stumble.

Kokushibo: "Good. You're taking this seriously. Now, once more. Only this time, I expect you to catch on to my patterns."
________________________________

Y/N, still reminiscing on the memory, readies himself. Kokushibo swings.

Y/N: (Right.)

Without a second thought, Y/N steps to the side before steadying himself, unlike his younger self who stumbled a bit on his recovery. Kokushibo attacks again.

Y/N: (Down.)

Y/N ducks before springing back into his battle ready stance as Kokushibo attacks again.

Y/N: (It's a feint. Right again.)

Y/N sidesteps the attack the same way he had before, this time with a counter swing. Obviously he felt nothing, but remembered how it was blocked before. It made him angry. Even now, could he hope to even come close to Kokushibo? His thought was interrupted as Kokushibo swings again, but instead of dodging like he would have before, Y/N blocked it, feeling something else connect with his sword.

Y/N jumps back and opens his eyes, only to find nothing but a small pile of blood. His blood.

Y/N looks down at his hand to see he was gripping his sword so hard his nails were digging into his palm. He quickly forgets about it as he closes his eyes again, sinking himself back into the memory of his training with Kokushibo.

Kokushibo swings downward and Y/N narrowly steps to the side, feeling the wind that had followed his sword, as if he were actually there.

Y/N counters again, which of course was blocked. But this time, he felt it. He felt the vibration travel from his sword to his arms as if he'd just swing his sword at another.

This time, Y/N decided to be offensive, spinning and swinging diagonally at Kokushibo, only for it to be blocked. There it was again. The same sensation. The same anger. The same longing to become stronger. He'd felt it almost all his life.

He didn't want to be called weak. He was tired of being weak. Tired of letting those around him get hurt because he wasn't fast enough, or strong enough, or brave enough.