To truly understand the Byakugan, Ryusei first had to define what a dōjutsu actually was.
The original series from his past life had never explained it properly; it only showed effects, never the structure behind them.
Through months of research, he was now certain that a dōjutsu wasn't just a random genetic mutation or a bloodline quirk.
It was a hybrid phenomenon, part biological organ, part spiritual seal.
In essence, a living chakra construct designed to store, shape, and manifest power into reality through perception, around the eyes, where it was the most logical.
The eye acted like a specialized projection node, both a seal and a converter.
It gathered and refined chakra within its structure until it could express unique abilities directly through sight, bypassing the need for hand seals.
That was why the Ōtsutsuki lineage, who had consumed chakra fruits from the God Trees, naturally manifested dōjutsu.
The sheer volume of chakra absorbed had to be stored somewhere permanent, and the eyes, as the fastest sensory interface with the world, became that vessel.
The Byakugan was the oldest and most balanced of those dojutsu eyes, the baseline 'template' of the Ōtsutsuki clan.
Its design favored adaptability and efficiency over intensity.
That explained why it appeared more frequently among the Hyūga than the Sharingan did among the Uchiha, and why its upper limits seemed lower.
The power curve was wide and even, not sharp and selective.
It wasn't 'weaker', just designed differently.
The Byakugan was evolutionarily refined to be universal, stable across hosts and environments, giving various classical vision 'buffs', and able to function in almost any condition.
That made it the foundation of the Ōtsutsuki's long-term survival and expansion, the eye suited for a species that colonized worlds.
The Sharingan, by contrast, was the opposite: a volatile, chakra-intensive offshoot created later, specializing in destructive potential, analysis, and illusion.
It demanded more chakra because it wasn't meant to be stable; it was designed for individual evolution, to push one user's eyes beyond normal limits rather than maintain consistency across generations.
The Rinnegan, and later the Tenseigan, were the higher evolutions of this system; of those two, the pure forms of godlike chakra sight.
Among them all, all the unique dojutsu variants from other planets in the universe, the Byakugan remained the ancestral "default "root, the original 'best' interface between chakra and the material world.
That was what fascinated Ryusei most.
The Byakugan wasn't just an eye.
It was also the Ōtsutsuki's original blueprint for how chakra should see reality, and, perhaps, how it could rewrite it, most naturally, in stronger forms.
After months of studying the Byakugan's properties specifically, Ryusei now began to notice something odd as well, regarding its current holders, on this plane, Hamura's descendants.
Its adaptability and layered nature reminded him of Hamura's Giant Tenseigan on the moon.
That massive eye was originally catalyzed by Hamura's own Tenseigan, yes, but why did the Moon's descendants keep sending their eyes to it generation after generation?
It couldn't be just a ritual.
The more he thought about it, the more sense it made.
The Giant Tenseigan likely grew through the same fusion principle he and Kanae were now experimenting with, multiple Byakugan merging, reinforcing one another, amplifying their collective power over time.
Because it simply worked easily.
And that similarity bothered him.
It wasn't just the fusion; it was the system.
The Hyūga's cursed seal practices on the Side Branch used an eerily similar structure to that clan on the moon.
Constant links of chakra, remote control, suppression, like a smaller, twisted version of that same mechanism.
The question came to him naturally later this afternoon as Kanae was, at the time, adjusting a chakra stabilizer.
"Where do the Byakugan go when a Hyūga dies?" Ryusei asked quietly.
She blinked, caught off guard, then answered after a moment.
"All Hyūga are buried in the same place. In the main compound's central burial ground, near the patriarchs' resting chambers. Even Side Branch members, strangely... It's… tradition."
"All of them?"
She nodded slightly. "Yes. That's how it's always been."
He let the answer sit for a moment before asking another question. "Have you ever heard of someone getting replacement eyes? For example, if a Side Branch member was injured?"
Kanae frowned faintly, thinking. "A few times. I heard of some who received new eyes because of injuries. But the cursed seal stayed. It didn't make any difference."
That was enough for him to start forming the picture in his mind.
It confirmed what he had already suspected, that the most likely way to beat the seal wasn't to remove it, but to overwhelm it.
To drown it in pure Byakugan energy, of your own, until the control link broke on its own.
However, that burial custom made him reach a much more dangerous conclusion as well.
The Hyūga almost certainly had an active central accumulation of Byakugan stored under the compound, passed down through generations, it seemed.
Maybe not just for replacements, or to preserve the strongest eyes across generations, for the main branch heirs, since Hyūga bodies especially could easily accept transplanted Byakugan, but for something far greater hidden beneath that tradition.
He started to see it clearly now.
The clan had probably built its own form of Giant Tenseigan, for some reason, a smaller, localized version beneath their grounds.
This is why they kept this illogical current clan structure.
To do that, but why? He still didn't know.
He just knew that such a structure, still without Hamura's personal Tenseigan as a core, would naturally be weaker, but since it was stationed on Earth, it didn't need to project across distance; the effect could be just as powerful locally.
That would explain everything.
It explained how Hiashi had sensed him that day from so far away.
After all, couldn't Toneri himself sense what was happening on the Earth from the Moon to an extent?
It explained the strange surge of power he had felt at that time from Hiashi's direction, and the feeling that some strange, unknown energy was helping in watching his movements.
It even explained how the Main Branch could maintain absolute dominance without ever leaving their walls.
He turned back toward Kanae, still calmly working on the tank beside him.
"Have you ever heard of anyone from the Side Branch trying to run away from that Hyuga Compound cage? I mean to other nations, for example? And what was the outcome of that?" He asked, voice casual.
She paused for a moment, then shook her head. "No. None that I know of. No one even tries that. Everyone just… accepts it's pointless, somehow. Even I did, now that I think about it. Despite the freedom that it would offer. We all tell ourselves the seal would activate no matter how far we ran, because of whispers of such rumors, honestly. So why even try?"
That settled it for him.
Even she didn't know about them; no one did.
Not a single case of escape or rebellion had ever been recorded, not even a rumor of a Hyūga in the underworld or living under a false name.
It wasn't fear that stopped them, most likely.
It was something deeper, mechanical, and absolute.
Those activation commands didn't only function in a close range.
Ryusei didn't say anything more.
He only nodded lightly and let the topic drop, pretending to go back to his instruments.
But inside, the chain of logic had already closed.
The cursed seals, the buried eyes, the central resting ground, it all connected.
The Hyūga had some kind of vessel underground, a lesser Tenseigan powered by the Byakugan of the dead and Side Branch seals.
Only the patriarch's direct line, men like Hiashi, could access it.
It likely had enough power to target enemies or Side Branch deserters, anywhere in the world. The seal wasn't just a tool for internal suppression; it functioned as an external anchor too, capable of channeling and transferring energy back and forth across vast distances.