The good thing about suddenly being hit in the face with a curveball? Being glad it came early, what if he decided to wait until much later? Join the academy and see the three of them and have a freak out in the middle of class? Not good for the reputation.
Instead what he did, was decide to become something just short of a beast. He began to cram his studies while he trained, now that he knew that even strenuous workouts wouldn't affect him at his young age, he decided to bring a new meaning to 'Muscles'.
The first thing he did was place suppressant seals on his body so that even if he worked out, his muscles would never grow past a certain point, it took almost a month to get them to work properly, to limit the amount of muscle and focus on its own personal growth without affecting his growth.
In short, it tore his muscles and let them repair instead of growing larger, so instead of getting larger, they got stronger in a smaller package, he would keep a lean swimmer physique until he hit his max growth, once he hit the prime of his body he would increase his muscle size and mass even more.
Was it painful? Very much so, but realizing you're close to a war considered to be one of the most brutal? A simple thing to get your mind firing on all cylinders.
Rei wanted to be a ninja god damnit, it was time he put in the effort to become the kind of ninja that could appear behind someone and claim they're already dead.
At first, his parents thought he simply became more focused on working out to try and show off to the future clan heads, Rei was always smart for his age, a genius in fact. So they assumed he had some reason for this, but it wasn't until his Mother actually caught him during one of his 'Routines.'
Rei was in the backyard, multiple seals placed on the ground around him, a massive log over his shoulders, his arms over it to keep it firm to his body, stones multiple sizes larger then he was placed at each end in what seemed to be handmade hammocks to carry them. Seals along with the logs, and even the stones. Only three seals actually around his midsection, one focused around increasing the load on his body, one to keep his body feeling like he was being thundered against by blows, and the third one being pain sensitivity increases. Increasing his sensitivity to pain mostly, but also other small parts, he needed to learn his body's limits, once he found those he would work on his mind and soul limits.
When his mother seen what she would describe as 'Hellish training.' she decided to speak with a friend of hers, Mito Uzumaki, also the wife of the first Hokage, grandmother to Tsunade, good friends before the both of them being exceptional kunoichi. When Mito had come near and stopped to be baffled by the many uses of seals she could sense, not just around Rei himself, but also the lands inside his clan compound, how even for her a decorated veteran at fuinjutsu. His uses, all be it small in scale, were so useful.
Watching him go through this hellish training lit a fire in her eyes, if he could work this hard, by his own self-study, why couldn't her Granddaughter? It was actually when she decided to extend a hand to Rei, to take him in as an apprentice, as a fuinjutsu specialist. How could she not foster a talent like this? Maybe he'd even be an able candidate to marry Tsunade in the future.
Rei of course, took this opportunity, now that he knew where he was, he could move a lot more freely, yes enemy villages existed still, but people like Danzo and Orochimaru weren't something he needed to worry about, and since he wasn't an Uchiha, the Hokage wouldn't dislike him.
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"So, boy, tell me the inspiration behind your uses of seals, namely the subtle confusion ones.''
Mito had got him locked in a room with him, like a cornered animal Rei was poked at until he finally spilled the beans.
"Well, think about it. Ninjas are supposed to be quiet, stealthy, and subtle. It is only because of the war, and other such things that made ninjas seem more like warriors, an army to send frontal battles against. Fancy ninjutsu that can light up a night sky, or abilities that can change the very terrain... None of these sound very subtle to me, but that also works to the advantage of those who can take it.''
He then pressed his hand to him. ''I'm just a kid, why would anyone worry about fighting me, let alone paying attention to me? Now, we add my seal that makes me an easier person to overlook, I don't stand out, do I? I look pretty normal, maybe in better shape than most kids sure."
He lifted his arms, he was dressed in a simple shirt with his clan symbol on the back, and shorts. Turning slowly. "I am but a kid, who can easily blend in the background, so unremarkable, you just overlook that I'm there.''
Mito's eyes flashed suddenly, her mind easily putting together what he was putting together, her voice full of pride, if children like Rei were being born, maybe fuinjutsu would take a massive leap in progress in the coming years?
"This idea of yours is incredible, and incredibly dangerous, listen. No one but those who already know is allowed to know about these seals of yours, alright? Fuinjutsu is a usual very well guarded secret, especially now with war brewing on the horizon. If you come up with stronger runes or new ones, you come to me and show them to me, alright? If in the case that you ever need to use them openly, I can help back you."
Rei nodded before he began to share with her all he had, the first thing she would do is get the gravity seals to the taijutsu specialist in Konoha, and she would be reporting everything to the Hokage, Tobirama. If Rei was this with just self-study and the occasional help from his parents, he may be a seedling to take special care of to make a strong pillar.
Mito wasn't blind to how the current Hokage was attempting to bring talented people in and make them feel indebted to the village to make it serve loyalty, and whereas she didn't like it, she didn't disagree with the method. Konoha would soon be at war, they needed strength, loyalty, and determination.
Her departed love may not like it, but even he would rather see Konoha survive the best it could, but would rather seek a way to see it done without bloodshed.
Mito then began to teach him the basics of Uzumaki seals and how they worked, how exactly to strengthen them, and how they worked. How to fit more seals in on small spaces, or to better use the space given, and even the start of how to write seals over each other to stack seals to be stronger, to add additional effects over another to make even more room.
In turn, Rei got to meet Tsunade, which despite her young age, was already a fierce demon in her own right, her strength, while nowhere near Rei's, was impressive for not actively training it as he did. Some might say his Youth was simply stronger than hers