Male OC X Kimetsu No Yaiba / Demon Slayer

Training

We're now 10 years old and Father decides to train us to become the future flame pillar... which I'll never be.

" My sons! Our family had been demon slayers for generations! Now, you two are going to continue our family tradition!" Shinjuro Rengoku really pumped up to train us. "First! You two need a strong and fit body! We can't fight demons if we're weak! Second! You need to flame up your heart so you can overpower your enemy! It also will boost your physical strength! Without wasting our time, physical training starts now!"

Phew, not Breath of Flame training. My moment of humility has not come yet. WAIT! PAY ATTENTION!

A wooden cane suddenly aiming at my head. With Atsushi's strength, I jump high in the air backward and do some air flip before landing swiftly on the ground.

Whoops

Maybe I shouldn't show off.

My father stares at me with unbelief while Kyojuro look at me with awe plus some sparkles appear around him. Shinjuro smirks. Oh no...

" Seems like you're a prodigy, Yoru! I have decided! Your training is going to be harder and more intense than Kyojuro!"

Crap. Just wonderful. If he knew I can't use Breath of Flame, his heart will totally shatter. What should I do?!!

"Heh. Give me your best. Show no mercy." Okay, first I must pass this physical training then I'll somehow disappear to somewhere before the Breath of Flame training starts. But... that means I have to leave Kyojuro. Sigh... I don't want to.

"If you say so. Don't regret it! Yoru! Swing that wooden sword 10000 times a day! Kyojuro! You do 1000 times a day! Don't bother the seasons! You must do it until the next spring!"

After teaching us the right stance, our hell of training starts. Of course, I cheated a little. I use a portion of Atsushi's power but not enough to turn any part of my physical into a weretiger. Even though I'm cheating, swinging the wooden sword multiple times is still hard. Now I know why the future Kyojuro has great grip strength. Wait, if my training is 10 times more worst than him, doesn't that mean I'll get 10 times stronger than him? Oooh... hunting demons would be easier! I can't wait to slay one!

I hope that I don't become bald like Saitama from One Punch Man anime because of my extreme training.

...

Months passed and I keep swinging my wooden sword at my yard with Kyojuro. My swings are getting better. I can swing 10 times faster than Kyojuro. So our daily training always ends up at the same time. He keeps praising me while I just pat his head and thanking him as a signal that I'm happy.

Now it's autumn. Beautiful orange leaves are scattered everywhere. The air is a little bit chilly but I don't mind that because Kyojuro's smile always managed to warm me. Ruka asks us to sweep the yard which now has scattered orange and yellow leaves.

"Otouto, let's have a contest. Let's make a large leaves pile. If you win then I'll sing a song for you. But if I win, I'll hunt down a demon tonight on my own..."

"Eeehhh?!!! NO!!! I don't want oniichan to get hurt! I'm gonna win!"

Heh. I would like to see you try. " Ready? Set,... Go!" We compete to pile up the leaves. Somehow my self-threat fired him up until he can catch up with my speed. This time, I'm not cheating for real. We collect leaves from the roof, the yard, and even from the road in front of our house. In the end, Kyojuro wins with a centimeter difference from my leaves pile. "I guess you win. Good job otouto."