Chapter 7

The Hashira he decided to train under was the Sound Hashira, Tengen Uzui. It was fitting since he was blind and could only rely on his hearing to figure things out. Since he was blind he's decided that he'd train himself to be able to see things even if he couldn't see.

So for the past year he's been practicing breathing techniques, working his body to the limit, and honing his swordsmanship. He wasn't some supernatural talent, but his dedication and passion was definitely there, that's why the Hadhira accepted.

With some hard work he'll be able to be a Hashira in virtually no time at all. Seikichi loathed the damn demons. One of their kind killed his family, his neighbors, his best friend, and destroyed the village! His soul desire to become a Demon Slayer had nothing to do with protecting other people from the situation he had, but revenge. He wanted all those human eating monsters dead, obliterated, annihilated from the face of this world.

Now, Seikichi recovering from an attack by a demon of that severity in only a week should be enough to tell you he isn't regular. Inside of Seikichi's body are tons of active RC cells, this type of blood is referred to as Marechi blood, because of its rarity and quality.

Seikichi and Kyoshi were the strongest kids in their village. Naturally, this was thanks to their RC cells being activated at birth and their body being able to handle it's assimilated use over the years as they grew older. It changes their body slowly and that's the main essence of RC cells.

The natural usage of these cells is to change the body over time, but there's more. While they are active, it's only default active, the person that holds them isn't manipulating them, which would give them more possibilities to explore.

When a person can manipulate their RC cells it tells you two things. One, that they have tons of them and two, that they are of high quality. Usually when a human gets too many RC cells in their body they would go under a mutation.

They would feel sick and lightheaded, dizzy, pains all over, then an intense hunger, and finally they'd turn into some grotaiouse monster with not the least bit of intelligence left in them.

Well that's if the quality is too great for the body, but the quantity still matters. The quality of RC cells also tells you a lot. Whether they can be manipulated, if your body can use them and lastly, if you can achieve full synchronization.

At the moment Seikichi's RC cell quality isn't amazing, but it isn't bad. He can manipulate them with practice, but that's about it. He'll only be able to get a boost in physical capabilities and that'll be all.

Not that he didn't know all of this.

The Hashira told him straight up, his hopes of becoming a Hashira are achievable, but he'll never have the right quality of RC cells to fully utilize them. Seikichi didn't care though, all he needed was to keep at his training and he'd be able to fight demons even if he didn't have RC cells.

Other than Revenge he gave a promise to his best friend that since he's dead, he'll take care of Hinā and make sure she grows into a respectable woman and hopefully find a good husband.

He swore on his soul that if a demon were to even harm her under his protection, he'd kill himself and the demon, so they both can burn in hell.

A/N: Crazy motivation for some kid.

…

"That…should be enough…for tonight." Seikichi said between gasps for air. He pushed himself up and used his bokken as a crutch as he walked to the door to the main house and slid it open before walking down the hall to his room.

He sat on his bed and grabbed his journal as he began writing in it. Today he was just at the HQ because the Hashira were called and he tagged along with Tengen. Tomorrow he'll be right back at the Sound Hashira's estate.

"I should take a bath." He mumbled as he smelt himself. Then again it was already close to midnight and he didn't want to wake anybody, so in the end he decided to just wait till early in the morning to take a bath before he started training again.

…

After Kyoshi had his things teleported into Muzan's estate, he got himself cleaned up and changed into something more fitted to sleep in. Muzan also changed out of a suit and into a regular kimono.

After getting changed Kyoshi went into Muzan's office with him and started helping with the experiments they would be doing.

…

"So the reason us demons can use Demon Blood Arts is because we have better manipulation over our RC cells?" Kyoshi asked.

"Yes, but only strong demons with better quality cells can have blood arts. That's why those lesser demons aren't as strong." Muzan answered as he looked at Kyoshi's blood under a microscope.

Muzan may be a cold-hearted, mass-murdering, manipulative, man/demon eating person, but that doesn't mean he didn't have his own interest and hobbies. A few being engineering, learning new languages, and most importantly chemistry.

To find a way to conquer the sun, he messed around with loads of things and ultimately developed a liking for chemistry. As he looked at Kyoshi's blood under the microscope, he grew confused.

Along with his RC cells and his new blood cells from becoming a demon, he still retained his human blood cells. With his eyes, Kyoshi was able to see this as well, but he didn't understand what he was seeing at all.

"Umm, Uncle Muzan. I don't get what those are." Kyoshi said as he pointed to the blood. He wanted to help with what Muzan was doing, but also didn't want to be a bother since he was completely in the dark about what he was doing.

"So you can see the cells with those eyes as well?" Muzan asked. Kyoshi nodded at him and Muzan got up from his desk and took a book from his bookshelf. It was a biology book which gave you the full rundown of what makes up living organisms.

He handed Kyoshi that and a dictionary in case there were words he didn't know.

"It shouldn't take long to read these." Muzan said as he went back to sit in his chair and started writing things down inside his journal.

Kyoshi looked around the office and noticed he didn't have a chair to sit in, so he left the room. Hearing the door creak, Muzan glanced towards the door and noticed Kyoshi had left. He was getting ready to see what he was up to when Kyoshi walked back in.

Although he was carrying a chair with him. Muzan sat back down and went back to studying while Kyoshi sat his chair next to Muzan's desk and read the Biology book with it in his lap while the dictation was on the arm of the chair, perfectly balanced.

As Kyoshi read the book with what he called his "Kakugan" activated he was able to read the book and memorize it and with his brain's enhanced processing speed, he was able to understand it and make connections as well.

Muzan could hear the low "ohh" that Kyoshi made when one of his questions was answered while reading. By using context clues Kyoshi barely needed the dictionary and after about 15 minutes he was done with the biology book. It only took this long because he kept having inner monologue's about possibilities with cells.

…

"If my human cells and my demon cells are different, shouldn't my human blood cells that are in my circulatory system have been broken down?" Kyoshi asked.

"Yes, they should've all been destroyed and replaced with the demon blood cells." Muzan answered.

"Maybe it's because of my RC cells. What if my body was already using my RC cells during my time turning into a demon and the demon blood cells just mixed with my RC cells?"

"Hmm, that would be possible. Well considering your body detected the demon blood as a threat until they mixed with your natural RC cells and started to divide." Muzan muttered.

"But wouldn't that mean my body was using RC cells before I was turned into a demon? I've never used them before though. I didn't even know they existed." Kyoshi asked with a confused expression.

"Maybe it was a subconscious thing for your body. Well for now it's impossible to tell. You have the body of a demon now and I can't pull cells from your blood individually to look at them so this is what we have to go off of." Muzan stated.

With that Kyoshi and Muzan continued in their experiments along with Kyoshi reading more about Biology. Muzan allowed him to read one of his journals since studying human biology was just the foundation for understanding Demon biology.

Kyoshi stayed in contact with Habuki and Keiko who were completely appalled at the stuff they heard. They didn't take Muzan to be the "caretaking" type, but over the days he even went with Kyoshi while he hunted and bought him some toys he liked. Faster than anyone wanted, a week passed.

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A/N: Kinda short chapter for my liking, but it's leading up to a time skip. I'll be switching between Kyoshi and Seikichi's POV for a while so you can see how both of them have been living while also understanding their way of thinking.

For the LI I haven't found anyone yet, but I do have an OC in mind. I was recommended somebody from the spider demon family, but after remembering what Rui's sister did and her personality. I remember why I hated her.

Should I do an axillary chapter with character sheets so you know who'd all be involved or would that be some type of spoiler?

Either way I already have her personality and backstory worked out to fit in and a scene of how they meet. I haven't added it into the ff yet, but the scene is already wrote out.