Isshin fell into a long silence.
Only silence could help him digest the shocking truths Shigure Sōsuke had just revealed.
What had happened to Shiba Kaien occurred in Soul Society long ago. Hearing about it now only served to expose Aizen's hidden schemes and the extent of his manipulations.
But the Hollow he himself had faced the one that had nearly killed him in the World of the Living was different.
That was something he had experienced firsthand.
The memory stirred a deep, simmering anger within him.
The stench of death… was not something one easily forgot.
Isshin had never provoked Aizen. He had never stood in his way, nor interfered with his ambitions. Yet, according to Shigure, during that battle, Aizen had likely been watching from the shadows.
Watching… and waiting.
That Hollow had not appeared by chance.
It had been sent to turn him into a Hollow an experiment.
Only due to an unforeseen variable Kurosaki Masaki had that outcome been avoided.
Instead, Masaki had become the one targeted.
Although it was through that incident that he met Masaki… and the two of them grew close, eventually becoming inseparable
That did not change the nature of what had happened.
Aizen had marked him as nothing more than a subject.
If Masaki hadn't intervened that day, he would have been Hollowfied… reduced to a mindless creature, or worse, a failed experiment discarded without a second thought.
Isshin's thoughts sharpened.
He recalled the turning point of that battle.
Just as he had begun to gain the upper hand against the Hollow, something unexpected occurred
A figure.
A man who appeared behind him without a trace… and struck him down with a single blow.
That attack had completely overturned the battle.
His injuries had deepened instantly, and the advantage he fought so hard to gain had vanished.
"…That person," Isshin muttered, his voice low, "was it Aizen?"
The possibility lingered heavily.
The Hollow had been powerful far beyond ordinary but Isshin had still been a captain-class Shinigami at the time. After a fierce struggle, he had managed to suppress it.
And yet, that single, perfectly timed strike had pushed him to the brink of death.
It was too precise.
Too deliberate.
Then another question surfaced in his mind.
"If Aizen was really there… watching everything," Isshin said, turning to Shigure, "then why didn't he intervene when Masaki destroyed that Hollow?"
He frowned.
"From what you've said, that Hollow wasn't ordinary. It should have been valuable to him."
Shigure paused briefly, considering.
"…Because that outcome may have been intentional as well."
"Intentional?" Isshin's confusion deepened.
Shigure spoke evenly.
"Aizen had already conducted Hollowfication experiments on Shinigami captains and lieutenants included. However, due to Kisuke's intervention, those subjects were stabilized using the Hōgyoku and escaped to the World of the Living."
At this, Urahara Kisuke, who had been quietly listening nearby, adjusted his hat slightly but said nothing.
"As a result," Shigure continued, "Aizen lost access to critical long-term data. His experiments were interrupted before he could fully observe their evolution."
Isshin listened intently.
"When you encountered that Hollow in the World of the Living," Shigure went on, "Aizen likely saw an opportunity to resume his research. A captain-class subject, isolated from Soul Society, would have been ideal."
Isshin's eyes narrowed slightly.
"A perfect test case," Shigure said plainly.
"But before that plan could be completed… Kurosaki Masaki intervened."
There was a brief pause.
"Masaki is not just any human," Shigure added. "She is a pure-blood Quincy an existence that has long been of interest to Soul Society… and, undoubtedly, to Aizen as well."
Isshin's expression hardened.
"After the Quincy were nearly exterminated centuries ago, their reappearance has always been treated with caution. Aizen would be no exception."
Shigure's gaze grew more focused.
"A Quincy's powers are fundamentally opposed to Hollows. For Aizen, the idea of forcing Hollowfication upon such an existence… would be an entirely new line of experimentation."
Isshin understood immediately.
"A Quincy… Hollowfied…"
The implications alone were staggering.
"Compared to repeating past experiments on Shinigami," Shigure said, "the prospect of breaking the natural incompatibility between Quincy and Hollow would have been far more enticing."
Urahara finally spoke, his tone quiet but certain.
"…And far more valuable."
Shigure gave a slight nod.
"Exactly."
He continued:
"In that moment, Aizen likely shifted his objective. Rather than recovering that particular Hollow or completing your transformation, he chose to observe what would happen if the target became Kurosaki Masaki instead."
Isshin clenched his fists unconsciously.
"Success would yield unprecedented data," Shigure said. "Failure would cost him only a single experimental Hollow something he is more than capable of replacing."
Isshin fell silent again.
This time, however, it wasn't shock that filled him
It was clarity.
Aizen hadn't hesitated.
He hadn't intervened.
Because to him, everything that happened that day… had already been within acceptable parameters.
Lives, outcomes, even unforeseen variables
All of it was just part of the experiment.