The sensation of cold water dripping all over his body gave Yahiko a refreshing feeling; something he didn’t expect he’d feel from cold water, as he wasn’t used to it.

15 minutes later, he was now in new clothing and headed to the living room . 

“Hmm! You look much better now! Come , sit next to me.” Said the Old man while patting the seat to his right. 

Obeying what his gramps said, Yahiko sat and focused his attention on the subject that would be addressed. 

“ As I've said before, Yahiko. This Old man will make you strong. And for that i’ll have to train you for a while but first let me just introduce myself again. *Cough* *cough*”

“ Your new grandfather is called Jing Huang. It’s been a while  now, but I used to be one of the strongest heroes of China! The level of the heroes in our country is at a higher level than Japan . Only All Might makes this country stronger in terms of battle potential. ” 

“ All Might ? The number one Hero was it?” Asked Yahiko as he remembered the articles he read on the web.  

“ Mhm. You should know about him. *Cough Cough* Anyway, the structure of our heroes' society would be a little hard to understand for a kid your age so that would be for later. What you should know is that we’ve developed unique techniques with the help of our 4000 years of martial arts history and advanced technology. But for you to be able to use them, you’ll have to work hard, yahiko. Do you understand? “ 

“hmmm.. so I have to work hard to use some unique technique but gramps will still teach me the basics right?” 

“ Hahaha, that’s right, you bright  kid! This old man has created a special training regimen for you to build muscle on that weak body of yours. Look at it. You can read right? “ Old Jing then tossed a piece of paper toward his new grandson. 

Looking at the content on the paper, Yahiko’s face slightly twitched : 

90 Push ups, Squats, Sit Ups, and at least 1 km run every day. ‘ Isn’t it too harsh for a 5 years old boy? Well, it certainly does work out for  me though.’ 

“ Don’t worry, you don’t necessarily have to do as much as is written. Just do as much as you can . We will also do one session of meditation each time before you begin exercising every morning. If after following this routine for a month you’re still capable of continuing , this old man will teach you the basics of his martial art , Then you’ll continue training that way until middle school. At that time, you might be able to learn this old man's unique skills. So, are you willing?” 

“ Yes!” 

“ Hahaha, good! I like your spirits! You can use the door connected to the dojo if you want to visit it. This old man will go meet an acquaintance. Be careful to not mess up the house, okay?” 

After explaining all he had to explain, Old Jing quickly left, leaving Yahiko alone in the house. ‘ So the Chinese have some kind of unique powers? I knew that the Old man wasn’t normal … he’s probably the one who defeated that crazy guy from before’ 

Yahiko followed the advice given to him and entered the dojo, discovering a rather large available space. ‘ I’m not sure how unique the powers they have are, but I doubt it would be as ridiculous as chakra.. ..Still, this is rather big.. Is the Old man teaching other people? ‘ 

The  dojo had  all the basic equipment most dojo had, but with some extra. Yahiko found a nice place to sit cross-legged and began thinking about the  jutsus he’ll be learning on his own. ‘Fortunately , i don’t have to wait for gramps to grow stronger on my own… no way i’m waiting until middle school. The first hurdle is my chakra reserves being too small. But I should be able to substantially increase it right?’ 

Chakra in Kaguya's reality was an energy created by the fusion of both physical energy and spiritual energy. The amount of chakra a being has can be augmented by increasing the amount of either of them. Physical energy, generally known as stamina,  comes from the body and is collected from every cell of the human body. It can be increased by physical exercises and anything or training that depends on moving the body for a good amount of time. 

Spiritual energy is acquired by activities that cultivate the mind like meditation, studying, or experiences. By increasing either one of those energies, one is not only able to increase the amount of chakra he possesses, but also the strength of said chakra. That being said, the amount and strength of chakra one is able to increase is limited by their genetics, and so, infinite growth is impossible… ..for most people.

Touching his chest and remembering what Kaguya said, Yahiko thought “ But that shouldn’t apply to me… According to Haruka-san, her core merging with me made my potential limitless. I should be able to increase the strength and amount of my chakra without limit…right?”

Next, Yahiko entered his consciousness and used his first ability to conjure the formula of multiple techniques that he remembered, using it as his own training regimen aside from Old’s Jing .

“ The Gentle Fist, The Strong Fist, and the Shadow Clones technique . With the formula of the Gentle Fist, the shadow clones and my Rinnegan , I think I should be able to gain near perfect chakra control in the shortest amount of time. And the training listed in the Strong Fist formula should be useful for my body. Hahaha , this is almost like a dream!” His eyes shined in excitement at his life to come, knowing that the possibilities at his disposal were endless. 

Quickly putting his thoughts aside, Yahiko began his grampa's training, followed by his own, in earnest, wishing for his chakra to increase as fast as possible.

With the help of his chakra, which could be used to increase all physical abilities to a certain level, he managed to train for such a long time that the sun had already set. 

“ Hahaha! This is ridiculous!” Exclaimed a rarely excited Yahiko. 

“ It’s only been a few hours but I can already feel my chakra reserves increasing! Is this also  because of Kaguya’s core? Let’s try something..” With a  simple thought, Yahiko activated his Rinnegan and saw for the first time the current range of his 360 field of  vision. 

“ 15 meters , huh? That’s not too bad for a 5 years old since this range will increase with training. Now let’s see further- huh? Gramps?” As he tried to see through the walls of the dojo to further test his eyes, Yahiko saw Old Jing sneakily looking at him through the dojo door. 

‘ Ouff.. Fortunately, he can only see my back.. ..I don’t want him to find out about my eyes yet… But… What is this? ’  As he looked at the appearance of his Old man through his Rinnegan, what he saw was a greenish energy circulating inside Old Jing's body. And by further concentrating his vision, the nature of this energy became clear to him ‘ Physical energy? I can actually see physical energy with my eyes? This should be really useful.. ‘

On the Other side though…

‘ Hohoho! This kid has some dedication… How long has he been training for?’ 

Opening the door to make his presence known,  Old Jing spoke “ Hahahaha! Never would i have thought that you’d still be here, kid!” 

“Huh, Gramps ?” exclaimed Yahiko, trying to act surprised. 

“ It’s fine now, you can stop training; you shouldn’t overdo it as you’re still a kid. Come, I've brought dinner. “ 

“ Understood”. 

After a quick bath to wipe off all the sweat and smell lingering on his body from training for god knows how long, Yahiko met his gramps  in the dining room, after smelling a rather appetizing smell. 

“ Ramen??” 

“ Yes, the friend I met  gave it to me. What’s with this look? Do you like it?”

“ *Gulp* .. ..Yes, very much so.” Said Yahiko while smelling the rich flavor in the Air. ‘ Cup ramen saved me a lot in my past life.. ..I'm afraid I got slightly addicted to it.. Bah! Who cares!’ 

“Hahaha, i see! Well let’s make this your welcome gift in your new house! There's a lot so don’t be shy.” 

The dinner was spent in harmony , with Yahiko devouring his food with a look of ecstasy and nostalgia on his face, which caused the laughs of  his gramps. 

“ Ahhh~... Thanks gramps, that was delicious! “ ‘ I guess I'll have to protect him from now on!’

“ I’ll take you with me once to meet that friend of mine, you’ll thank him yourself by then. This Old man is gonna take care of you, so that you in turn take care of him in his old age! Dying alone is a horrible feeling..” suddenly said the old man with a melancholic look on his face. 

Seeing such a face on his new parent figure , Yahiko had an understanding feeling ‘ I guess everyone has his own story… ‘ 

“ Don’t worry, gramps . I’ll take care of you as much as possible.. ..When I get bigger.”

Feeling that the words of a kid no older than 6 years old slightly brought him comfort, the Old man quickly tried to hide his feelings “ Ha! You’re 50 years too early to think about that, kid. But I do appreciate it, hohoho! This Old man will remember those words.” 

And so, the life of Yahiko with his new grandfather continued. Every morning was filled with intense training that would’ve left a normal kid his age in shambles long ago. While behind the back of his Grandfather, Yahiko quickly learned and mastered various jutsus while understanding more and more of his Ocular powers.

Animals on whom he tested his ocular genjutsu frequently visited, and sometimes accompanied him to school.

It was in this fashion that days, months, and years passed, as he became more and more powerful, with almost no one aware of  his existence…

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Additional bonus information: 

Gentle fist : The main martial art of the hyuga clan in the NarutoVerse. It requires incredibly precise chakra control to use effectively. The hyuga clan made use of their byakugan to target the tenketsu of their opponent, precisely sending the right amount of chakra through those tenketsus in order to either seal them or speed the flow of chakra. As this taijutsu sends the user's chakra through the opponent's body, it’s also possible to easily cause internal injuries with the slightest tap. The gentle fist style techniques’ names all start with “Eight Trigrams *name of the technique*” .  So if Yahiko says or thinks “ Eight Trigrams”, you know he’s about to use a gentle fist technique. This will be one of Yahiko Main’s fighting styles.

The Strong Fist : The main fighting style of Rock Lee and Might Guy. It’s main principle is using powerful strikes to cause severe external damages. This fighting style also has multiple techniques attached to it. Also one of Yahiko's main fighting styles. 

Shadow Clones technique: This jutsu basically allows you to create real clones that you can actually touch. Each clone created divides your chakra proportionally so it can be dangerous. The clone can use most of the jutsus of the original. One of the best uses of shadow clones is by using them while training, as the original can gain the experience of all his clones after training, thus increasing his learning speed. 

I_Am_L_72

Time skip... I want to begin canon as soon as possible but there are some bases that need to be built first so i hope you don't mind too much.