Male OC X Kimetsu No Yaiba / Demon Slayer

Isekai

 ??? POV

 "Huwaaaaa! " A sound of crying infant fills the room.

 "Congratulations Ma'am! You got another son! Here you go."

 "Ara ara~ He's blessed! " I hear my mother say behind the thin wall.

 "Is everything alright?!! " Father barge into the room while I follow after him.

 "Yes, our son is safe and sound. Look dear, he looks just like our first son! " Mother exclaims happily and gives the infant to father, who carries him in his strong arms. I take a peek from below because I'm short but I'm still unable to see him.

 "Ah! You're here! Congratulations! You're a big brother now!" The maiden congratulates me. Father gives the infant but not before telling me to be careful.

 As I hold the infant, he stops crying. He opens his eyes... They're just like mine. Maybe this is how I used to look when I was his age.

 A couple of months later, my brother said his first words. But rather than 'Kaa-san' or 'Tou-san', his first words is ..... 'Oreki'. I have no idea where he gets that word from. He sounds so sad when pronouncing that word.  Sometimes, his tears starts falling but he didn't wail or anything. Just silently crying. Is it even possible? Even the kids a few years older than him will bawl out whenever they're upset.

...

 After the guest left, I take a peek into my brother's room. He dislikes guests so he'll just lock up in his room or secretly playing in the forest during the daytime. I see he's drawing something and I've got curious.

"Hey otouto! What are you drawing?" My brother just give me a cold shoulder. That's kinda disappointing.

 Before I leave, he finally replies, "A beach." Interested, I enter the room uninvited and sit down across from him. Between us, there's a beautiful drawing. "Woah! I never knew you could draw!"

"You don't need to know." His reply is cold as usual.

"This place is called 'Beach'? It's beautiful! I've never been there before! Wait.....when did you go there?"

"As I said, you DON'T need to know." He crossed his arms.

" Eeeehhh? You're so mean! What am I supposed to know then? "

He stares at me for a few seconds. "Fine, nii-san. I'll tell more about the beach." He points to several parts of his drawings with his pointer finger. "Alright. Listen carefully. This is called 'sand'. It's different from the ground. It has unique golden tint color that varies in different places. This is salty seawater. Usually, the color will change according to the sky. It's where the saltwater fish lives. And here, we have the fishermen's village. Their income is mostly from fishing. And this bird is called a seagull. And this weird animal on the sand is a crab. It lives in separate holes under the sand with its species. Each species have their own 'territory'. Uhh...... I'll stop there."

My interest spikes up even more. "Whoa! That sounds so cool! How do you know those?!!"

"I'm not telling." He crosses his arms again. I pout. My eyes scan the drawings again and caught something interesting.

"Who are those two?" I point towards two black figures sitting on the sand, far from the village.

"That's... me and a friend of mine." He turns his head away.

"Hmm? Do you have a friend? How lucky! What type of person is he?"

"... His face is emotionless. He gave off this eerie vibe that scares people away. That's why he's lonely all the time. But he's actually a nice person."