Tobirama Senju personally classified the Yang Seal as an S-rank Forbidden Jutsu and had it entered into the Scroll of Seals.
Most jutsu end up labeled as forbidden either due to their severe casting conditions or because of potential bodily harm. In the case of the Yang Seal, its effects on the soul remained unclear, and its long-term risks were unknown, it would require time and testing to verify. Thus, it was placed under the "forbidden" category.
But Rihito Sato suspected another reason:
Tobirama probably just didn't want this technique spreading within the Uchiha clan, in case it led to a surge of Mangekyō Sharingan users and destabilized Konoha's power structure from within.
Uchiha Kagami, however, was no fool.
He knew full well what his clan was like.
He was cool-headed, rational, and eerily similar in temperament to Tobirama, that was precisely why the Second Hokage valued him.
As the only Uchiha Tobirama ever truly acknowledged, Kagami didn't need to be told what to do.
A simple glance from Tobirama was enough, he understood everything.
The other Yin Release families were easier to manage.
A single S-rank mission reward was enough to satisfy them.
They were only brought in to assist Uzumaki Mito in developing the seal, providing insight into the subtleties between Yin chakra and the soul.
But the actual seal formation? They had no idea how to use it.
Even if you placed a fully drawn sealing formula in front of them, they'd be clueless without years of dedicated study.
Kagami, on the other hand, had learned sealing jutsu directly under Tobirama.
With his Sharingan, he could see the intricate flow of chakra within seals.
With Mito and Tobirama's guidance, he mastered the Yang Seal.
Rihito, of course, had been present for the entire development process.
He was there from the moment the idea was born, watching as the technical roadblocks were overcome one by one, absorbing it all like a sponge.
He felt no fear toward forbidden techniques.
After all, this was his own idea.
And from what he could tell, the structural integrity of the Yang Seal was flawless.
So he began training it immediately.
A blood-red seal bloomed on his forehead, perfectly round, resembling the divine mark of a Buddha.
Uzumaki Mito watched with mild concern, but only just that.
Rihito was always within her sight, if anything went wrong, she could personally intervene and forcibly seal the technique with a Dual-Seal Formation.
And truth be told, Rihito had already mastered all of the Uzumaki clan's sealing techniques.
Even if there were small flaws in the Yang Seal, given two or three years, he could fix them on his own.
During this period, Rihito also grew closer to Uchiha Kagami, who turned out to be quite talkative once you got to know him.
Through one such conversation, Rihito learned an interesting fact:
Uchiha Kagami's son was already attending the ninja academy.
His name? Uchiha Mitsuteru.
Currently in the fourth year, and Rihito's senior.
Eight years old.
Rihito decided that with nothing better to do, he'd go check out the father of Uchiha Shisui and see just how capable he really was.
School days are boring as hell, you've gotta create your own fun sometimes.
So one afternoon, Rihito sought out a few upperclassmen and, after a bit of "friendly persuasion," he finally located Mitsuteru on the training grounds.
To Rihito's amusement, Mitsuteru looked exactly like Shisui, same hairstyle, same features. Three generations of copy-paste models.
Definitely blood-related.
When he found him, Mitsuteru was sparring with a boy from the Sarutobi Clan.
As was typical for ninja clan kids, both of them were already using basic family techniques.
At age eight, the gap between clan and civilian kids had already begun to widen.
Under Rihito's watchful gaze, the two exchanged blows, strikes, kicks, chakra-infused parries.
Occasionally, they even tossed out a minor jutsu or two.
It reminded him of Kakashi and Obito's old academy scuffles.
But chakra was limited at that age.
Once they finished trading jutsu, both quickly ran out of stamina and stopped.
It was only then that they noticed Rihito standing nearby.
"Hey! Who the hell are you, brat? Who let you in here?"
The Sarutobi kid glared at Rihito's plain clothes, no family crest, no clan markings.
Must be some civilian kid trying to spy on their techniques.
Instant hostility.
But Rihito wasn't the type to back down.
"I'm here for Uchiha Mitsuteru.
Not you.
Now get lost."
"Wha, you little punk!"
The Sarutobi boy rolled up his sleeves, ready to throw down.
But before he could charge, Mitsuteru calmly grabbed him by the collar.
"Shinnosuke.
We just burned through most of our chakra.
Attacking him now wouldn't be wise."
Mitsuteru's voice was level and composed.
Once the situation was diffused, he turned toward Rihito.
"I don't know you. What do you want?"
"I heard the Sharingan was the most powerful dojutsu in the world,"
Rihito said casually.
"So I came to see it for myself."
It was a lazy excuse, but what else could he say?
"Hey, I was bored and wanted to mess around"?
Mitsuteru's expression softened slightly.
Being an Uchiha meant constantly dealing with nosy people curious about your eyes.
He was used to it.
"Sorry, kid.
I'm only eight.
My Sharingan hasn't awakened yet.
Come back when it has."
That was clearly just a polite brush-off.
But Rihito had no plans to wait years for a rematch.
"No problem.
You've used up most of your chakra anyway,
and I'm not the type to kick someone when they're down."
"But tonight, 7 PM.
Training Ground 13.
Let's see if the Uchiha are really useless without their fancy red eyes."
Mitsuteru's forehead twitched slightly.
This little brat really knew how to push buttons.
If he refused, rumors would fly,
The Uchiha were already an unpopular clan with few allies.
And if word got out he backed down from a challenge, even his fellow clansmen would chew him out.
"Tch.
You're one annoying kid.
Fine.
It's settled."
Satisfied, Rihito nodded and turned to leave,
Never once sparing a glance at Sarutobi Shinnosuke, radiating unfiltered arrogance.
Shinnosuke looked like he might explode.
"I HATE kids like that!
Stupid cocky little brats!"
Mitsuteru awkwardly nodded in agreement.
After all, they were still friends…
You had to back your people up.
<150 P.S = 1 Extra Chapter>