(Yo, Future Me here. I have read quite a bit of this certain trope from different authors, but it's basically just an OP Sun Breather. However as I thought more about it, if the character is truly a threat to Muzan, he'll just hide until the MC dies since he has all the time in the world. It's what happened to Yoriichi, at least according to headcanon, so I lost interest and stopped writing after coming to that conclusion. Enjoy if you can.)

((P.S. None of this is edited or proofread before uploading.)

This story will be OC-Centric, but it will be closely tied to the main character of the series.

The OC will be the twin brother to Tanjiro Kamado, so some things are bound to change from this.

There are a few warnings I want to put up before you start reading.

This is an M-rated fic for a reason. Expect adult themes. I'm not sure about pairings.

The OC will be strong from the beginning of the series(Manga Chapter 1/Episode 1).

The OC will not be a carbon copy of Tanjiro. There would be no point, in case you were wondering.

Manga spoilers will be apparent in this story. Do not read ahead if you don't want to be spoiled. At the time of writing this, I'm caught up on all (198) chapters.

I will not be adjusting ages, so Tanjiro won't suddenly be 16 at canon.

With that out of the way, I hope you enjoy the story.

Chapter 1

Difference in Death

Tanjuro Kamado isn't normally a nervous person, but having your first child would bring on that effect.

What's more nerve wracking is when you find out that you're not only having one child, but two on the same day.

Twins are not common and are seen as a gift which both Kie and Tanjuro took as a blessing.

Their first child came and they have already decided on a name, Tanjiro. The second child was unexpected and they found out a few months ago. A name wasn't something they have come up yet so they decided to wait until the child was born.

They thought that the moment when they saw their second child's face, they would know.

The second child came after a few minutes and Tanjuro was shocked beyond comprehension when he saw his child's face.

He was identical to his older brother, Tanjiro, all except one feature.

A mark across his forehead and going down to his left eye. Its color is the same as his own burgundy-tipped hair.

What many did not know about Tanjuro is that he's heard of the stories of his ancestors from his own father. A man with hanafuda earrings, a flaming mark across his forehead, and the dance of fire as vibrant as the sun to go along with it to save his ancestors.

The Hinokami Kagura was a dance to thank this man for allowing them to be alive today, and when he saw his youngest son, he knew that this was physical proof that he's watching over his family to this day.

Proof that he lives on centuries after his death.

He knew what to name his child.

"Yoriichi Kamado". He spoke handing the child to his wife.

She smiles and nods.

"Yoriichi Kamado. I couldn't think of a better name". She holds her child while Tanjiro has already been left to rest.

I've always felt different from my twin brother, Tanjiro.

He and I are identical in appearance but strangers in personality. Well identical except for a mark which I have tried to ask about, but dad said I wasn't ready yet. Whatever that meant.

I'm currently 4 years old and I feel distant from my two other siblings. Tanjiro and Nezuko.

I've always been more observant, calculating, and overall just a quiet child.

My father and mother are sometimes worried about what I'm thinking, but I don't have any problems with interacting and playing with my siblings or parents.

Emotions are hard for me, but being with my siblings helps. I just don't know why I have this problem in the first place.

I'm not only vastly different in mindset, but a mark across my head to illustrate the difference.

Why was I born this way? Why couldn't I be more like them?

I feel love and compassion. I can show it and express it, but it's hard. Rare even. Maybe it'll get easier as I grow up, but why is it this hard? It seems natural for them.

It's always been like this, but to take it a step further, I see things that my siblings don't.

I have asked Tanjiro and Nezuko if they see the things that I can and they have both answered no.

I'm able to see muscles, organs, and blood vessels. It's always been natural as breathing to me but I soon realized that wasn't normal when I asked them.

I felt that there was something wrong with me. That I was sick. It would explain everything.

Around this time I decided to exercise that will hopefully dispel this sickness.

Training.

I didn't want to worry my parents so I thought that training and strengthening my body would cure my disease.

Dad has been getting pale and sick the last few months so I don't want to add to his burden, so I will take it upon myself to fix myself.

At first I was going to start light and slow, but I soon realized everything was incredibly easy.

I wasn't getting tired or exhausted, but my muscles were hurting which was strange to me. Running around was the most taxing, my endurance seemed to be pretty good from the times I've played with Tanjiro and Nezuko. I was always the fastest. They always couldn't keep up, and I always never understood why. We are similar in age.

Push ups were something I expected to be hard on my body but I was able to do 20 before dropping. Again, it wasn't exhaustion, but pain, which is something I don't really understand. Why wouldn't I be tired yet? I decided to do more exercises.

Sit-ups were next and it was about the same before I laid down in exhaustion. My breaths were short and I started to feel pain everywhere. I couldn't even think of why everything started to hurt all at once. My first thought is to control my breathing and I take in deep breaths until everything goes back to normal.

I blinked. What just happened? The moment I felt my breathing correct I felt relatively fine except for my sore muscles, but still…

My vision still didn't go back to normal, in fact I felt that it's getting more clear and accurate. Is this really a sickness?

Everything was slower and I became more aware of my own body than before.

I could go back to normal and see everything normally, but why could I see such things in the first place.

My thoughts began to move away from this being a disease.

I have always felt different compared to my siblings, from this mark on my face, to my personality.

Maybe this vision is from my mark and that's why my siblings couldn't see what I can.

Because they don't have it.

My father has something similar on his forehead and I decide that I'm going to talk to him about it. After my training routine, of course.

It was after breakfast that I decided to go outside.

I dressed in a red and black patterned haori with an orange kimono underneath. I also had white sandals and a black hakama.

This training routine has only been going on for about a week and not much has changed. I can still see these things that my siblings cannot.

I also notice that it's getting more clear and I'm more aware even if by a minimal margin.

This deduction is why I am veering away from this vision being a sickness.

My training is all physical from running and body-weight exercises.

Push-ups, sit-ups, and squats is what I do.

During this week, I realized I had a lot of fun doing this training and I enjoy it greatly. There are times where I feel pain all over but once I calm my breathing it lessens to fatigue. It's addicting. Sometimes I go overboard and train more than I should as a young kid. I'd go on for hours and even use playing with my siblings as training.

This effect has brightened my mood and I'm able to more easily interact with my siblings. It really did cure whatever I was going through in a short time. I may not be at their level, but I'm much better off than I was a week ago being emotionless most of the time

I'm not sure if being able to do this level of training is normal for my age and I'm guessing it isn't from judging my siblings laying across the ground panting when we are playing, but that's why I'm going to ask dad about that.

There's a new exercise I came up with which I use my arms to hold my body up from falling from the ground. It focuses a lot on my stomach area. That's what I'm doing now and I feel it's definitely working.

"What are you doing, Yoriichi?" A voice from behind the treeline asks as my father's figure comes into view.

"Training". I respond standing up from my position.

He looks surprised.

"Why?" He asks, coming closer. I blinked and began thinking.

"I first believed that I was sick, so I thought training my body would make me better, but now I'm not so sure". I admit and he now looks confused.

"Sick? Why would you think that you're sick?" He asks in a gentle tone.

"I can see things that my brother and sister couldn't. I can see muscles, organs, and blood vessels if I focus, and I become much more aware while everything feels slow". I explain and he becomes visibly surprised.

"I see. I can tell you that you're not sick. This state that you go in is something I didn't expect because it requires pushing past your limits and extreme training. I'm also able to do it and it's called the transparent world". He explains which greatly relieves me.

"Transparent world?" I ask and he nods.

"It's a state of being. It's about looking at what's necessary. Like closing your eyes to listen more closely. You take out what's unnecessary. This is only possible through intense training and breathing". Tanjuro explains which confuses me at the end.

"Breathing?" I ask.

"Let me ask you this. When you were training did you ever feel tired?" He asks simply.

"No, but when I kept going for a while it began to hurt, but it wasn't exhaustion, only pain". I respond to which Tanjuro pieces it together.

"Then I believe I have your answer. It seems that you are already able to efficiently breathe by instinct. This way of breathing can be taxing on your body which is why you feel pain if you exercise and keep this way of breathing". He explains. 'Although, I don't know how he's able to enter the transparent world. It doesn't make any sense'. Tanjuro thought in concern.

I nod.

"Would it be better to change how I'm breathing?" I ask to which he shakes his head.

"It would be more detrimental to do that. We are at opposite ends of the spectrum. I had to learn how to breathe correctly and train my body to its limit while you can already breathe efficiently but it's harder to train your body at the same time. We may start at different points but you will reach the same destination as I did". He explains and I'm left confused.

"I only started training to get rid of what I thought of as a sickness, but now I'll probably keep training anyways". Tanjuro looked mildly amused.

"You like training?" He asks and I nod my head.

"It's fun for me and I could also play more with Tanjiro and Nezuko". I smile at him.

"Hmm. Let me ask you a question. If you were to become strong, what would you do with that strength?" he asks curiously.

I raise my eyebrows and I think about it for a while.

"I would help you with selling charcoal since I'd be stronger to cut wood and burn it. I would also protect my siblings. The strong should always protect the weak". I answer confidently.

He smiles.

"That may be true, but you have me and kaa-san to protect you". He responds interested in his son's response.

"Well, you wouldn't always be here, so I have to help too. I don't want to rely on anyone for something that I could do. It's the job of an older brother to protect the younger siblings, so I'll become the strongest so that no one could hurt us". I state with determination and his eyebrows raise.

A proud smile comes across his face.

'He seems to resemble that man greatly. Not only in his mark but also his personality. I see his kind hearted nature and his sense of responsibility that no 4 year old child should have, but there's more. He wants to be accepted. I never would have thought that he'd be going through so much this early. Perhaps he is ready, although he's much younger than I was'. The thoughts end there as he looks down to his son.

He rubs my head.

"You already have such a strong resolve. That's a good quality to have as an older brother to want to protect your siblings, but you know you're not the oldest, Tanjiro is". He laughs a bit at the end.

I pout and look down.

"Only by a few minutes". I grumble a bit and he laughs a bit more.

"Back to the matter at hand, I could help you in your training". I look up in curiosity.

"Really?" I ask excitedly.

"Well would you like to learn the Hinokami Kagura?" He asks and to which I widen my eyes.

"That dance you did at the beginning of the year?" I ask and he nods.

I remember that dance like it was yesterday. Dad has always done that dance each year to ward off diseases and to pray for good health, but it was different for me. I felt connected to the dance as if I had a personal connection to it. I was too young to remember it before, but this year I see it clearly.

It felt pure and calm. I felt my body heating up as I saw it. Like my body recognizes the dance.

I nodded my head and my training began at that moment.

Tanjuro smiled.

The next year would be hellish. My body was exhausted every day, but I enjoyed it. Now that I realized why I felt pain instead of exhaustion, I could push myself to further my efficiency in breathing. I was scared that I would tear or break something, but I soon learned my limits. It was a hard balance to find since I couldn't actually stop unless the pain became too great.

My father had me increasing my stamina and endurance so that I could train my muscles more in the future. I wouldn't be doing weight training for years but it's good to prepare for it and my breathing would definitely go down in efficiency if I didn't do these endurance exercises.

He had me dodging projectiles and while that may seem extreme, I'm able to do pretty well especially if I enter the transparent world, but that's incredibly taxing to do for an extended period of time, so I usually do exercises with and without it.

Stretching was another part of the training since the dance would require a great deal of advanced movement that I simply couldn't do if I was stiff as a board. It still hurt, but it got better with time.

During this time Tanjiro and I would play and grow closer together. Nezuko would always stay by mom's side and we would all play together every once in a while.

I wondered if Tanjiro would want to join us, but I didn't think he'd like this training. It's exhilarating for me, but he probably wouldn't take it like that.

At first, I couldn't really think of a reason why I loved training so much other than it's fun to do, but I later realized that I liked the progression. Being a stronger person than I was yesterday is something I never felt before and it soon became addicting, so I pushed myself farther.

Another form of training that my dad had me do was recovery training. I would try to attack him with a wooden sword and he'd just throw me around. He wasn't even armed. I had to try and land on my feet and be able to fall in a way that I could quickly get up into a stance, although he hadn't really taught me any stances. He said that didn't matter for now.

The training was brutal at first since I had to get used to exercising and breathing at the same time.

My body wouldn't hold up learning the dance on breathing alone, so my dad had me exercise to a certain point which took about a full year which is understandable. I'm learning the dance at a much younger age than my father did so I can see why he wanted me to be as physically strong as possible so I wouldn't hurt myself.

I asked my father if Tanjiro was ever going to learn the dance and he said that he's still too young and that he doesn't breathe like him and I. That confused me and I wondered why he can't breathe like dad and I.

Am I always going to feel this different? I'm beginning to lean towards yes, but if it's to protect my siblings then I believe it won't matter. They accept me. That's all I need.

Currently, I'm 5 years old and my dad, one day, asks me a question.

"Yoriichi, why do you want to train so hard? I know you have said that you want to protect your siblings and that you have fun, but why do you keep going at such a rate?" He asks and I think for a while.

"I want to protect my siblings to the best of my ability. Training everyday will make sure they won't have to worry, so that they don't have to go through what I am doing. The strong protects those that can't protect themselves. Strength for selfish reasons would lead a lonely life. That's why I train everyday". I look up to him and he smiles warmly.

"I'm proud of you". He held on to me and I closed my eyes. "I believe it's time to learn the Hinokami Kagura. You're at the point where I can teach you the forms."

I nod in his chest and the next few years go on like a blur.

We advanced to swordplay and he would spar against me, and I didn't stand a chance. He's said that he was never a swordsman but his technique would make you think otherwise. It was like he was born to wield a sword and I wanted to be like that too so I constantly practiced. My father has stated that I'm a prodigy in the way of the sword. I just understood what to do and how to wield a sword incredibly well, but there are things that I have to work on. I asked him about it and he said that he'll tell me one day.

I didn't really accept this answer but I couldn't do anything about it.

I finally started learning the Hinokami Kagura. He would teach me one form and I'd try to use it at a proficient level, but they were extremely difficult on my 5 year old body. There were 12 forms in total to learn and it seemed like a monumental task to master all of them.

I've noticed that my father's health has been declining over the course of the 2 years of learning these techniques and I help him whenever I can. Tanjiro could also tell. I think his sense of smell is starting to really develop, able to sniff out my worry and fear that our dad may die soon, and I'm hoping that it won't happen.

My sense of smell is also on par with Tanjiro's although he has said that's all he has. Sometimes I would hear things that he couldn't and see things in such detail that he couldn't see. I wonder if this is because of our difference in breathing. Dad said that when you start breathing effectively then your senses grow stronger and since I have always breathed this way, my senses are more advanced.

Although, one day when Tanjiro can also breathe like I do, his sense of smell will be greater than mine.

It took about 2 months to just learn one technique to proficiency. At this rate it will take 2 years for me to learn all 12.

I just have to keep working at it.

Another 2 years went by and our family is growing. Takeo, Hanako, and Shigeru were new siblings that I have grown to care for. Shigeru was born last year and another sibling is on the way. It was also around this time that Tanjiro joined us for training.

Our dad isn't as active as he once was when I was training with him so I helped out whenever I could.

"So you've been training for a few years?" Tanjiro asks when he's about to start going through training.

"Yeah. It's been 3 years since I started. I just learned all the forms of Hinokami Kagura". I explain and he looks amazed.

"Wow! What do I gotta do first?" He asks and I almost flinch at what he's going to have to go through.

"Physical training…" I state in a deadpan voice as our father just chuckles.

"Yoriichi? What's wrong with physical training?" He asks but I don't respond as I remember the horrors of that training. It may have been fun and addicting, but it was still difficult.

"...Yoriichi?" He asks, slightly nervous now.

One day when I was 9 years old my father led me away from the house. This wasn't anything new since we usually went out here to teach me the different forms of the dance, but it's different as we reach a shrine that I've never seen before.

He gestures to me to come inside and it's a small room that'll fill about 10 people on the floor. My father does something surprising and lifts one of the mats that reveal stairs downward.

I follow him down the stairs into a new room that would change my life.

It's a nicely decorated room which is filled with notes and messages, presumably from ancestors, but what caught my eye is a sword with writing that covers the entire wall in front of me.

"This room has been in our family for generations and it has messages from our ancestors, but what I wanted to show you is this on the wall". He explains and I finally read what's written.

Yoriichi Tsugikuni. Our Savior. Sumiyoshi Kamado and Suyako Kamado. Here lies his sword. The blade that cuts through all demons, even Muzan Kibutsuji cowered in fear from Yoriichi's might. To those who may one day wield the sword. You must abide by Yoriichi Tsugikuni's will. Protect those who cannot protect themselves, slay Michikatsu Tsugikuni, and slay the progenitor of demons, Muzan Kibutsuji. If you cannot dedicate your life to his will, do not dare wield this sword in battle.

"You're named after this man who saved our ancestors. I am showing you this because of your progress. I have nothing left to teach you. You have exceeded all my expectations and I believe you're the one who may finally achieve his wishes". My father explains while I'm in awe.

"Demons are real?" I ask and he nods.

"For as long as anyone can remember they have roamed the world. This man was a demon slayer and he was extremely powerful. When the time comes you may also become one. You have a strong resemblance to him. The mark on your forehead is one he shared. What will you do?" He asked and I already knew my answer.

There is evil out there and this man saved us. It would be wrong to ignore his wishes after what he has done for us. I wonder why he wants one of his relatives killed. It could be that this person is also a demon.

I never asked my father why he didn't enact our savior's will, but he didn't seem like the type to do it. He cared more about living peacefully and with our family. I smile at that thought.

I move forward and grab the blade unsheathing it.

It was a black blade but the second it was fully unsheathed it turned red. I felt the connection with this blade instantly.

"I will do it". I respond simply as if it wasn't a monumental task. He smiles.

The next few hours were of him showing me how to clean and properly tend to the blade that he has been doing since his own father showed him.

About a year later is when everything changed.

My dad died. He died peacefully from his sickness, but he died nonetheless. It hurt a lot.

I was always an emotionless child at a younger age, but it's gotten extremely better to where I feel like a different person. This is the first time I have ever felt true sadness.

Pain is all I felt.

A feeling I never wanted to feel again.

Everyone was feeling grief alongside my 6 siblings and mother, but I felt like I have died myself. I felt empty at that moment.

I had no idea how to express myself in this situation. I just stood there as we all gathered to say our goodbyes.

I didn't say anything.

I didn't accept this.

I cried and wept. I started to shake uncontrollably and my family tried to hold me and calm me down.

I didn't want to feel this ever again.

So I kept working and trained to death.

It was at this moment I vowed to protect everyone.

I would be the person my dad was proud of.

He left us his earrings and the Hinokami Kagura when he passed. His wish was that we continue to pass down these earrings and the dance to our own children one day and we of course agreed.

There was only one set of the hanafuda earrings so we each got one. Tanjiro had his earring on his right ear and mine was on the left.

My mother never really found out about my training other than I was learning the Hinokami Kagura with Tanjiro and I feel somewhat bad for leaving her in the dark, but I know she would have been worried if I did tell her the extent of what I was doing.

He left me with the sword of our savior and his will to fulfill but there was also one more gift he gave us in a form of a lesson. A lesson I wouldn't forget.

10 days before my father's death.

"So there's a bear around here?" I ask and our father nods.

"Yes. I've placed bells in the surrounding area if it comes too close". He responds as both Tanjiro and I follow.

"How come Yoriichi and I are here then?" Tanjiro asks with confusion.

"I want you two to see something. You both have been training in Hinokami Kagura and I wish for you both to witness it in combat". He explains leading us through the woods.

The 3 of us head there and there's a giant bear against the strung up bells.

"That's the bear that killed several travellers. I can see why". I state with a slight grimace.

Our dad nods and he takes out an axe and in an instant the bear's head was rolling in the snow.

It was so fast, but I saw that he made two cuts with his axe.

"Are you okay? I've been thinking that I can take over doing the Hinokami Kagura next time. I know that your health has been declining". I offer but he just smiles walking back towards us.

"Thank you Yoriichi, but I haven't felt the strain of the Hinokami Kagura in a few years. It's strange that when I learned it from your grandfather that it was much harder, but as time went on, I learned to eliminate unnecessary movement while exerting the most strength. What's important is proper breathing and control, when you do all that, your head becomes gradually clear". He explains.

"Gradually becomes clear?" Tanjiro asks.

He nods.

"In the beginning you must learn the movements and sensibilities. You open your five senses, becoming conscious of every single blood vessel. This process is incredibly strenuous and you will begin to feel a wall. After you have learned to cut away anything unnecessary you will have what's necessary in your movement and close yourself". He looks at both of us.

"Close?" I ask with confusion.

"Think about it like this. When people listen closely, they close their eyes right?" He asks and we both nod. "Just like that you focus on what's necessary. And before long, you are able to open and close blood vessels as quickly as blinking. Once that happens you are able to open an illuminated path. This world is known as the Transparent world that is only accessed when pushing your body to the absolute limit". He explains looking at me specifically.

"Wait, but how-" He interrupts me.

"Yes, you are able to access this world. I don't fully understand it, but you are able to enter this state. I've said before that you must undergo intense training but it seems that you've always been able to do it and it has even improved". He explains.

It probably has something to do with this mark. I was born with the ability of entering this world and absolute control of my body. This is probably how I was able to quickly acclimate to the Hinokami Kagura.

At first it was pretty hard to stay in but it's been getting easier and much more clear.

I can now see blood flow in finer detail and joint movement, even predicting attacks to a certain extent.

"Cool! Could I go there also with you guys?" Tanjiro asks in excitement.

"If you work diligently and push yourself then of course you can". He smiles, rubbing Tanjiro's head.

Dad has always been like this. I felt no change from when he cut off the bear's head and how he's acting now. His temperamental never changes.

He's like a plant.

No ill-will even in combat.

My sense of smell has gotten a little better over the years and I can sense that nothing changed from before and after combat. There wasn't any anger, hatred, bloodlust, or drive from when he took his stance from when he cut the bear's head off.

No battle presence.

Is that our lesson? I will have to think about this more.

Now that I think about it. He knew where the bear was without the bells making a noise.

The years have gone by again and I'm now 13 years old along with my brother, Tanjiro. I've grown a lot and I'm quite surprised that I'm now standing at 5'8"(172.74cm). It's unnatural since I'm about as tall as my dad at such an age. Is it from my training? Maybe that has a part in it.

When dad died Tanjiro and I had to take over the responsibility to provide. I did a lot of hunting for food and helped Tanjiro with selling charcoal. Around the winter time is when it gets harder, but I usually assist Tanjiro during this period.

We had more food readily available and while we were still poor, it wasn't as bad as it could've been. My training has a practical use that's eased the burden on Tanjiro and I.

My brother didn't really join me for training much since he saw no point in it. He even spoke about his concern about how hard I push and I told him that it's better to be safe. Tanjiro didn't really ask about it again and he'd join in on a few runs that I'd go on, but that's about it.

He never did understand my obsession for training. I also kept my sword a secret while practicing. My mother would have been quite angry if I'd been swinging a sharp sword around at 9 years old.

I didn't really go into the village at all except for a few times a year. The times I did is when I didn't bring my sword since I didn't want to draw attention to myself, but now that I'm confident in my abilities I might start, although my mother will find out… not like I could keep it a secret for much longer.

Dad never told them about the shrine and about demons. I understood that, since he didn't want them to worry about demons prowling around at night.

He must've had his reason so I kept silent about it unless they asked me directly.

"Tanjiro! Yoriichi!" My mother calls out.

"Your faces are pitch-black, come here". She wipes away at our faces.