The story goes back many years, when Higashino Shuuichi had only just begun working under Aizen. After conquering Hueco Mundo with ease, Aizen quickly realized that the Menos Grande were hollow shells—far from the terrifying beings the Shinigami had spoken of in Soul Society.
What he thought would be a new frontier turned out to be nothing but disappointment, and his interest in Hueco Mundo vanished.
He turned his gaze back toward the Shinigami.
At that time, Aizen firmly believed that only by further excavating the depths of Shinigami power—reaching places that could not even be looked upon—could he surpass Yamamoto Genryūsai.
So, he decided to use one of Higashino Shuuichi's Shikai abilities, Shōheisha (Commander), which controlled people, as a model. From it, he developed a technique for Hollows that could extract reishi from Shinigami instead of consuming them outright. This would make research far easier.
But after using the ability a few times, Higashino realized its fatal flaw: Shinigami were not faceless souls in Rukongai. Any Shinigami's death attracted immediate attention from the Gotei 13, tightening defenses in that area.
This made Aizen's harvesting highly inefficient.
So, in order to prove his value, Higashino proposed a refinement—if Aizen could alter the technique so that it could be used without his Shikai, then Higashino could infiltrate more easily. At the same time, Higashino requested a transfer from the 10th Division to the 4th, using its medical duties as cover to extract reishi unnoticed.
To avoid raising suspicion under Unohana Retsu's watchful eyes, he even suggested reducing the amount extracted each time. The victims would still function normally, though diminished.
This method yielded smaller harvests, but in steady trickles. Given the sheer turnover of ordinary Shinigami, nearly all would eventually be touched by his blade at least once.
It was this very suggestion that allowed Unohana, nearly ten years ago, to notice something strange—yet she could never pin down the cause.
Only two years ago, when she happened to see the 4th Seat of the 12th Division, Sarugaki Hiyori, reviewing old case files, did she discover faint traces of a pattern. Following that thread, she began to suspect Higashino.
But her suspicion had only just begun when, in a casual promise to Yamada Seinosuke, she accidentally alerted the snake.
To this day, she has only suspicion—no evidence.
That was why Aizen had said her current contradictions were thanks to Higashino's careful measures. If he had ever left more obvious traces, Unohana would have struck at him years ago.
All this, however, was not explained by Aizen directly. Higashino pieced it together himself, using data Aizen provided.
For Aizen, uncovering Unohana's past actions was trivial. The young women of the 4th could never resist his cursed charm.
"But Aizen-dono, even so, Captain Unohana remains a hidden threat. She hasn't acted against me yet, but her suspicion won't disappear. If this continues, unless I stop working for you, something will break!"
Higashino set the files aside, voicing his worry.
If it were possible, he wouldn't mind if Aizen found a way to eliminate her.
Any affection he might once have held for characters of the "original story" had long since faded after living in this world for over a century. Now, only his own safety mattered.
But Aizen, the master strategist, had just slain two captains, enraging Central 46. With several captains still chasing Baraggan in Hueco Mundo, he had no desire to stir up further trouble.
"Then confront her."
Aizen's tone was flat.
But to Higashino, the words struck differently.
"I… don't understand, Aizen-dono." He was truly baffled, once again unable to read what medicine Aizen kept hidden in his gourd.
"In the short term, we can't move against Unohana. So the solution is simple. Go to her and tell her you intend to replace the vacant 10th Division captaincy. See her resolve.
"If she is determined to keep you under watch, like Hirako Shinji with me, then we'll have to adjust our approach. But if not—then your time and qualifications already make you more than fit to be a captain."
Aizen explained calmly.
Higashino stood opposite him, emotions tangled.
Hearing this, how could he not suspect that Aizen had already planned this back when they eliminated those two captains in Hueco Mundo? Perhaps even earlier, back during the Noble Hunt incident.
Had Aizen been eyeing the captain's seats all along?
The more he thought, the more plausible it seemed.
After all, the captain's positions were fixed—one seat, one role. Unless a captain died or stepped down, no one could replace them. The sole exception was the 11th Division, but that position never suited Higashino's cover. Aizen would never assign him there.
And Aizen himself was watched relentlessly by Hirako Shinji. For him, a sudden surge in power would only bring instant retaliation.
Moreover, most captains rarely left Soul Society. Killing one inside its walls was far harder and riskier than in Hueco Mundo or the World of the Living.
So Aizen's current proposal was clearly one of convenience, a natural extension of circumstance.
To Aizen, was Unohana a problem? Of course not. She was only a problem for Higashino.
Thus, he could use the opportunity to both test a long-term objective and weigh Unohana's strength—all with Higashino as his probe.
Two birds with one stone.
But for Higashino, it was torment.
Aizen didn't know Unohana like he did. She would never let him out from under her gaze.
She would "care for" him, just as Hirako kept "watch" over Aizen.
In other words, an inevitable, one-sided battle.
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