...
The Bureau of Technology Development is a new department.
It was only established after Kisuke Urahara took over.
There are even more bizarre people here than Sosuke Aizen.
From people with horns on their heads to those wearing strange golden masks flailing about... These are still considered the "normal" ones.
Naruto even saw a green-haired catfish sitting at a computer and operating it.
Their attitude towards Kisuke Urahara is rather strange.
There is none of the respect a subordinate would show a captain, as they are all absorbed in their own work.
Urahara doesn't seem inclined to greet these people either, leading Naruto and Shihōin Yoruichi to the lowest level of the Bureau of Technology Development.
A vast, empty, and desolate expanse.
Naruto is somewhat surprised.
There is such a massive space underground, even with an open sky - it's as if he has entered another world.
He also feels puzzled. Why was he brought here?
This place seems more like a training ground. If it were for a medical examination, the upper levels with the precise instruments would be more suitable.
"Young Uzumaki, let me introduce this place to you." Urahara says proudly, spreading his arms wide. "This is the first result I achieved after establishing the Bureau of Technology Development."
"Here, we can evade any detection from the Seireitei."
"Including... the restrictions the Central 46 has placed on us captains."
"Even if we were to release our Zanpakutō, it would go unnoticed by outsiders."
Naruto furrows his brow.
This... sounds less like "research" and more like something else.
"The seal on you is quite remarkable." Urahara says, drawing closer. "Be it Shinigami, Hollow, or Quincy..."
"Even the powers used by the human spiritual mediums in the Human World are vastly different."
Urahara pauses, observing Naruto.
He hadn't initially planned to interact with Naruto.
Seeking Isane Kotetsu's assistance was just a normal collegial favor.
But...
As his research into the "seal" progressed, and the information he gathered from Sosuke Aizen about Naruto...
He finds himself unable to resist the urge to investigate the mystery surrounding Naruto.
It bears an uncanny resemblance to a certain technological development he currently has in hand.
He continues, "That type of technique is generally a shallow, superficial application of spiritual power control, but it is remarkably clever in its bodily development."
"The effects it presents are no less than our Shinigami techniques."
"It's a very unique approach and perspective, an area we've never explored before."
"If only I had more than just the transcripts Lieutenant Kotetsu provided me..."
Naruto lifts his shirt "Do you want to take a look?"
"I don't mind."
"No, no, no." Urahara quickly shakes his head. "The external forms hold no meaning for me. What I want to understand is the core mechanism."
"So..."
"Fight me!"
Naruto is bewildered, the transition too sudden for him to react.
"Fight, release your Zanpakutō and fight." Urahara's voice is firm, enthusiastic. "To the point of losing control."
"I will record all the data to analyze the seal on your spiritual power."
Naruto hesitates a bit.
Yoruichi places a hand on Naruto's head "Kid, if you lose control, it just means you aren't familiar enough with your power yet."
"The more you use it, the more familiar you'll become!"
"And don't worry about harming us, despite how Urahara looks, he is a Captain after all."
Urahara smiles "And here, we are not constrained."
Naruto nods his head, drawing his sword "Then please, Captain Urahara, Captain Shihōin."
As he chants the release, the ominous, ferocious spiritual power of the Nine Tails surges forth.
Soon, four tails manifest, and he loses rationality, going berserk and frenzied.
When he next awakens...
Naruto finds himself in a hot spring.
Shihōin Yoruichi is naked beside him, her body bruised and scarred. Seeing him awake, she greets him cheerfully "Ah, you're up? You really went all out, huh."
She is very frank about it.
Naruto is embarrassed, blushing and quickly averting his gaze.
He knows a similar stealth technique to what Yoruichi is currently using, but... having her in the same hot spring is a different matter altogether.
The young man has never seen such a sight before.
Urahara is crouched by the hot spring, looking at Naruto with shining eyes "Have you recovered enough? Can you release it again soon?"
"I've collected some interesting data, but it's not comprehensive enough..."
Yoruichi angrily splashes a handful of hot water at him "Think of me too, my stamina can't keep up with this brat."
That day, Naruto lost control and released four times.
Urahara is thoroughly satisfied, and makes an arrangement with Naruto for twice-weekly testing sessions.
As for the compensation...
It's the full results of the research on the "seal", as well as Shihōin Yoruichi's "free" training lessons.
As the physically strongest among the Twelve Captains, her guidance is naturally of great benefit to Naruto.
Two months later, Urahara unveils his first achievement.
On the foundation of Naruto's original body seal, he has added two more incantations.
The effects are remarkable.
It allows Naruto to maintain his rationality even when transformed into the Four-Tailed state, and to only lose control when reaching the Five-Tailed transformation.
The "Five-Tailed" form brings Urahara great excitement.
A white bony mask appears on the face of the berserk Naruto.
At first, Urahara thought it was a Hollow's "mask", but upon cutting off a piece with Yoruichi's help, he finds that it is not. It is simply "bone", completely different from Hollow power.
But it still gives him some new ideas for his research.
However...
The power of the "incantations" seems limited to this. No matter how many "auxiliary techniques" he carves, he is unable to keep Naruto rational in the "Five-Tailed" state.
Urahara was quite anxious. He was eager to see what the "Six-Tailed", "Seven-Tailed"... even the "Nine-Tailed" form would be like.
This sword called the "Kyūbi" is far more powerful than he imagined.
During this period, it is mostly Shihōin Yoruichi sparring with Naruto, and Urahara occasionally joins in person to collect data.
The two reach the same conclusion:
In the "Five-Tailed" state, Naruto is already at the level of a top-tier lieutenant.
If he could reach the "Six-Tailed"... he would likely be at the captain-level, roughly equivalent to the newly promoted Captain Rōjūrō Ōtoribashi.
Extrapolating from there, the limits of just his "Shikai" alone could potentially place him among the top captains.
However, as eager as Urahara is, he knows his limits.
Whenever Naruto is about to lose further control and manifest an additional tail, Urahara unhesitatingly intervenes to seal and cut off the flow of spiritual power.
While ensuring Naruto's safety, as well as his and Yoruichi's own, he continues his research.
In the training, learning, and research, time flies by quickly.
...
Academy's training grounds.
Sosuke Aizen is teaching Naruto Kido spells above number fifty, when he suddenly remarks "It's graduation season again."
Naruto is momentarily taken aback "Ah, yes, graduation season."
He looks towards the distance, where the cherry blossoms are in full bloom once more.
"Have you decided which Division you'll join?" Aizen asks.
Naruto shakes his head "Isn't the assignment done by the academy after application?"
"That's for regular students." Aizen smiles. "You're different, you're a prodigy."
"Captain Hirako wants to recruit you."
Naruto contemplates "The Fifth Division, huh..."
"But speaking of which, Naruto, you're supposed to go to the Human World for Soul Burial training with the sixth-year students in a few days, right?" Aizen changes the subject, seemingly not wanting to burden Naruto with difficult choices.
Naruto nods with a grin "Yes, Headmaster Hogei Ichiji told me about it."
"Soul Burial" is the Shinigami ritual to cleanse and guide wandering souls to the Soul Society.
It is a skill every Shinigami must master.
Naruto is no exception. He looks forward to the internship in the Human World.
Maybe... it could help unravel the mystery of why he keeps inexplicably traversing between "Konoha" and the "Soul Society".
The ritual is simple, the Human World internship is more of a "symbolic" representation, signifying the students have finally stepped out of the academy and can stand on their own as full-fledged Shinigami.
"I wish you success." Aizen offers his blessings. "But with Captain Urahara and Captain Shihōin personally instructing you, Naruto, I'm sure you'll have no problems."
Naruto smiles even brighter "And with Lieutenant Aizen's guidance as well!"
"If not for you, I wouldn't have been able to learn so many advanced Kido spells!"
"Just call me Sosuke, we're friends after all." Aizen's tone is gentle and unhurried. "I'm just a lowly Lieutenant, how can I compare to those two captains."
"But... Naruto, your progress these past months has been much faster than before. What did those two captains teach you?"
Naruto scratches his cheek apologetically "Sorry, Sosuke, but Captain Urahara instructed me not to tell anyone about that."
Aizen apologizes "I shouldn't have pried. The captains' teachings are hard-won, Naruto, you must cherish them."