(2 years later. Third Person POV.)
"Sun Breathing: Dance!" Kai exclaimed as he tried to swing his wooden sword downwards.
*BATHUMP*
Kai felt his heart strain under the Sun Breathing and dropped his sword, he quickly fell but chose to remain still on a plank position.
And here's the reason.
Masaru opened the slide-door and spoke to him.
"Are you okay Kai? I heard a loud noise, did you fall again while practicing your swordsmanship?" Masaru asked.
"N-no…I was just focusing on my planks here and my sword fell from the tree, see?" Kai gestured the sword which was indeed a few centimeters away from the tree.
"Oh, whew, if your mother knew I let you take a hard fall again she will skin me alive." Masaru sighed in relief.
It's been 2 years since Kai's diagnosis was made, not much as changed actually, kids at preschool didn't bully him or anything, at worse they pitied him but that was all. The biggest change is that Kai finally has a "proper" sword.
It all started with him "finding" an swordsmen-like movie to watch, he acted like a kid wishing to see it with all his might and his parents complied.
What happened next was something similar to what Bruce Wayne experienced when watching that "El Zorro" movie but no one dies here. Kai convinced his parents that the movie inspired him to become a hero with swords and his parents, not wanting to crush his dreams, agreed.
And with that Kai entered a fairly decent dojo to train his swordsmanship, of course he was learning Kendo more than the proper breathing techniques but martial arts start at foundation so this part was important for Kai.
Why not learn other martial arts you ask? Because he already had some of those memorized in his past life...also because Kaizo from my One Piece Fanfic uses mostly martial arts and this is a demon slayer fic.
Cough, cough, anyway, Kai has trained his techniques fairly well, but there's still one big issue.
Total Concentration Constant.
And I don't mean that he has an issue maintaining it anymore, the issue is the complete 24 hours, for obvious reasons he can't maintain it while he's asleep, all for these next reasons:
He can't maintain it while sleeping. Even if the breathing is interrupted he doesn't have a way of waking up to correct it. There's no imaginable way for him to ask "hey, could you monitor me while I sleep and wake me up when my breathing pattern changes?" that comes out naturally.
Now you can see why its hard for our boy here to learn the complete Total Concentration?
Sure, it might be just a few hours but here's the twist, the breathing techniques help the body in several ways, including recovery, this was a little bonus from Wexof, yes for everyone who learns them not just Kai.
Now, let's imagine a situation in which Kai's unconscious and his breathing is halted, his recovery gets slower and he wouldn't be able of returning to his hero work as fast, which in a real fight a few of those seconds could define life and death.
With those details clear the reason why he's faking to do some planks is because he doesn't want to worry his family whenever he fails, they're a little too worrisome and tend to exaggerate a little so Kai has to keep a distance in these situations.
Kai panted on the floor after his father left and sat at the base of the tree. He controls his breathing to cool off and sighs. The entire training he's done has been harsh, especially for a 6-year-old, but it has been quite rewarding, how? He managed to push a water bottle that was 10 meters away with an air slash.
And before you say something like "oh, it's not a big deal" bitch please look at the math:
A small, single-serve water bottle, the kind most people drink from, usually holds between 250 and 330 milliliters, which means it weighs roughly a quarter to a third of a kilogram when full. It's light, but not weightless, and pushing it any real distance using nothing but air is much harder than it sounds.
To knock a bottle of that size ten meters away with a sword swing, the air displaced by the blade would need to move extremely fast, around 15 to 20 meters per second. That's not a steady breeze, but a sudden, concentrated burst—closer to a sharp pressure wave than ordinary wind. Because air is very inefficient at transferring force to solid objects, the swing itself would need to generate somewhere in the range of 400 to 800 joules of energy, focused tightly forward.
For context, an average person swinging a sword produces about 100 joules. Well-trained practitioners might reach 200, and elite athletes using full-body rotation can sometimes push close to 300–400 joules. Going beyond that means the character is operating outside normal human limits. In a story setting, this implies enhanced physical conditioning, exceptional coordination, and techniques that allow the body to transmit power with almost no loss.
And Kai's just 6 FUCKING YEARS OLD, what were you doing at that age, me personally was scared shitless of Skull Greymon for reasons I still don't understand to this date.
Summarizing: Kai's REALLY strong as a kid and with great power comes great control to not break anything so that's why he still's a tad unaware of how strong he really is.
And yes, his proficiency with Wind Breathing has indeed increased. Kinda obvious isn't it?
But that's not the best part of today, but tomorrow is the first day of Elementary School…yay…
That means no more of the same stories over and over again, they'll actually be teaching something relevant to their lives, somewhat and most important of all, Kai managed to remain with his group of friends, dubbed very cringy by Tei as 'The Gang', all to the same school.
Which was a nice touch to Kai's life.
Kai decided to take some time to rest as he decided to focus on his main study subject: Toxicology. Or poison research however you prefer to refer to it.
Insect Breathing was created by Shinobu Kocho as a compensation for her lack of strength in Flower Breathing. Techniques that rely heavily on poisons to deal major and fatal damage to the enemies.
Kai of course doesn't plan to off any guys so he'll have to do with the typical paralyzing, sleeping and mildly damaging poisons for the techniques to work.
Now you might be wondering, why focus on Insect Breathing when it's just a nerfed version of Flower Breathing? The answer is simple, because it's a good balance between the nimbleness of Flower Breathing and the fluidity of Water Breathing.
Every breathing technique has its uses, its up to Kai how and when he'll use them during situations to get an advantage.
Speaking of advantages, I guess it's time to talk about the elephant in the room: The Mark.
Now Wexof tweaked the dynamic of the Mark a little bit, but all he gave Kai was a little note in his mind that said: "Mark won't kill you no more but it'll give you lots of debuffs afterwards", he said.
To make the explanation simple The Demon Slayer Mark brings the user's body far beyond the usual limit, surpassing superhuman, due to the excessive circumstances one has to be in order to awaken the mark and then maintain it, after its use the user will fall under the next symptoms: Weakness, muscles pain, bone pain, artery pain, high blood pressure, migraines, sensible eyes, balls pain (if male user), constant nausea (if female), loss of appetite and finally insomnia.
Yeah, a pretty fucked up list of debuffs that happen post use of the mark, as for the lasting of them I'd say around 5 days a minimum and unknown days after, it all depends on several factors like the current body condition, the condition pre-use, and overall damage sustained.
Even so Kai has no desire to reach the mark yet for obvious reasons, I'm pretty sure a kid his age's heart would blow up if his blood pressure rose that high and his heart beat that fast.
The Transparent World and Red Blade are still FAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAR off for him to even graze with his current skills so let's not talk about that.
Once Kai rested enough he changed his breathing technique and repeated, from the fluent moves of water he changed to the powerful slashes of flame to the sharp edges of wind and to the fast strikes of thunder.
Yes, the other thing Kai's learning right now is Breathing Changing, the very same Tanjiro did when he didn't have enough strength for Sun Breathing so he intertwined between Water Breathing techniques as well.
This is going to be extremely important since amidst one attack Kai will be capable of changing techniques and surprising his opponent(s) with the swift changes. For now the only issue is that Kai seemed to get more tired per switch, which something he's hoping to overcome by the time he's a teenager.
Speaking about teenagers, it's time for the first day of Elementary School…I know it doesn't have anything to do with it, it just seemed appropriate.
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Sorry for the lateness, I accidentally sat on my glasses and can barely see shit, gotta wait for my parent(s) to return from Cartagena at the end of the week to buy a new pair so, sorry.