Soul Society.
Nanatsuki arrived early at the Technology Development Bureau.
The corridors were quiet — most researchers preferred working at night.
Normally, Nanatsuki wasn't one for early mornings either, but today, he'd received a message from Kurotsuchi Mayuri saying there had been progress in his research, so he came to check it out.
The Bureau used a flexible work system — there were no fixed hours, as long as everyone met their annual work quota. Since Shinigami lived for centuries, they didn't measure time the same way humans did.
As Nanatsuki walked down the hall, he bumped into a petite Shinigami carrying a paper bag of breakfast. He recognized her — one of Mayuri's research assistants, known for her diligence.
"Good morning, Captain!"
"Morning, Chiyo," Nanatsuki replied with a nod.
"Captain, would you like some breakfast? I just bought it from Yoshinobō!"
She shook the bag cheerfully.
"Thank you…" Nanatsuki smiled and shook his head. "But I've already eaten. Is Mayuri inside?"
"Ah… he's still in the lab," she said, looking a bit disappointed.
She'd bought the food hoping to share it with the Captain.
Nanatsuki continued on toward Mayuri's laboratory.
After the 12th Division was restructured into the Technology Development Bureau, Nanatsuki had redesigned the layout — a central hall connected to several specialized project labs.
Urahara Kisuke and Mayuri each led their own project divisions and could use a full laboratory's worth of resources.
Mayuri's lab was two levels underground. Nanatsuki took the lift down.
Inside, several oddly shaped researchers were busy at work — Mayuri's taste in assistants leaned toward the eccentric.
"Captain!" they greeted, pausing their tasks.
"Carry on. I'm just here to look around," Nanatsuki said, waving a hand.
He could already sense Mayuri's Reiatsu deeper inside, so he walked straight toward it.
Mayuri's current research focused on Reishi shells and soul structures.
Many of the Reishi shells salvaged from Miyamoto Tetsuya's lab had been transferred here.
After walking past rows of cultivation pods, Nanatsuki found Mayuri.
The man looked nothing like when he'd first been freed from the Maggot's Nest — his face was now covered in black and white makeup, his ears plated with gold, and a bizarre cone-shaped hat sat on his head.
Mayuri's aesthetic sense was… distinctive. Nanatsuki couldn't help staring for a moment.
"Oh~ no wonder the air felt colder all of a sudden — the Captain's here!" Mayuri cackled, breaking his focus.
"Any new discoveries?" Nanatsuki asked.
"A small breakthrough, yes," Mayuri replied, visibly excited. "Come, take a look!"
He plucked a small pellet from a cultivation chamber and handed it to Nanatsuki.
"What's this?"
Nanatsuki sensed it — and frowned. "There's emotion inside this Reiatsu?"
He'd thought it was just a condensed Reishi core, but it gave off faint traces of emotional resonance, like that of a living soul.
"Ahahahaha!" Mayuri laughed. "As sharp as ever, Captain."
"After all, lifeless matter and souls are both made of Reishi," he said, stroking the chamber's surface. "But while we can replicate the structure infinitely, the creation of consciousness itself — that's the domain of the gods."
"And the key difference between the two?" Nanatsuki asked.
"Emotion!" Mayuri's eyes glinted. "Only souls capable of emotion possess true awareness!"
"I studied Hikifune Kirio's prototype for Artificial Souls, and the later Gikon Capsules others made. But they either simulate a soul with Reishi — lacking true individuality — or modify an existing one."
He gestured proudly to the pellet. "But when I combined Reishi shell data with Hikifune's research, I succeeded in creating this — a Temporary Soul imbued with fear!"
Nanatsuki nodded. "Incomplete, but still a leap from zero to one."
Unlike Reishi shells, which cloned data from existing Shinigami, Mayuri's Artificial Souls were made from nothing — new entities, born without origin.
That difference made the process far more difficult.
"And this is the first of your Project Sleep experiments?"
"Not yet," Mayuri said. "It's still too primitive. Fear alone doesn't make a soul — so this one doesn't deserve to be called Sleep-01."
The "Sleep Project" was Mayuri's dream — to create life from nothing. He had named it so because one must dream to create.
After explaining his findings, Mayuri led Nanatsuki to another pod.
Inside floated a human body identical to Mayuri's.
"This chamber simulates the environment of the human world," he said flatly. "Inside is the prototype of your concept — the Gigai."
Nanatsuki had entrusted him with developing the Gigai system — artificial physical vessels that would allow Shinigami to blend seamlessly into human society.
If the Bureau manufactured them, they could also monitor all Shinigami activity in the World of the Living.
"You've progressed quickly," Nanatsuki said, pleased. "Can it be deployed yet?"
"Not until testing's done. Once I see the data, I'll decide whether it's ready for production," Mayuri replied, shrugging. "But I'll need your authorization to visit the World of the Living."
"I'll come with you," Nanatsuki said, raising a brow. His instincts told him Mayuri's goals went beyond testing a Gigai.
"Heh…" Mayuri froze for a second, realizing he'd been caught. "I only want to retrieve some research samples, that's all."
"Quincy materials, perhaps?" Nanatsuki said. "You're a sly one — even the Onmitsukidō didn't find them."
"I admit it," Mayuri smirked. "But since the Captain's interested in Quincy research too — shall we go together? It'll take a day or two to prepare your Gigai."
Two days later.
Nanatsuki and Mayuri stepped into the World of the Living.
"This is my first time here in a human body," Mayuri said, flexing his fingers uncomfortably.
"Hmm… feels too tight — like wearing a bodysuit. The restriction's unpleasant."
Nanatsuki nodded. "The Gigai feels stiff… and my spiritual perception is dulled — like a veil over my senses."
"And," he added after a moment, "the sensory range is incomplete. You implemented sight, hearing, and smell… but no taste or touch."
Despite the rough feel, the Gigai 1.0 model was functional enough for field use — interaction with humans and normal movement were fine, though sensory feedback needed refinement.
"Are taste and touch that important? Fine, I'll note it down," Mayuri said, scratching his head. Then he frowned. "But Captain, there's a problem."
"You can't exit the Gigai on your own, right?" Nanatsuki guessed.
"Exactly. Should've brought a Gikon Capsule. Without it, we'd have to destroy the body to leave — which means no test data."
Nanatsuki chuckled. "Good thing I came prepared."
He produced two Gikon Capsules and handed one to Mayuri.
In his previous life's memories, Shinigami used these to exit Gigai — so he'd brought some along.
"Oho~ Captain, you're always three steps ahead!" Mayuri laughed and swallowed the capsule.
Instantly, his spirit separated from the Gigai.
"I have a body! I have a body!"
Mayuri's Gigai jumped up and down gleefully — like a child with his favorite toy.
The capsules were made by other researchers, using Temporary Soul technology to fill the Gigai with self-sufficient Reishi constructs.
"Seems to work perfectly," Nanatsuki observed.
Mayuri's expression darkened. He punched the Gigai hard in the stomach.
The body convulsed and spat the capsule out, collapsing into an empty shell.
Mayuri re-entered it calmly.
"No matter how many times I see it, it's still fascinating," Nanatsuki said.
"A Reishi body can inhabit a physical shell — and through spiritual pressure, even interact with matter."
In this world, spiritual beings weren't intangible ghosts — they were Reishi-based lifeforms. They could bleed, age, and die, yet still remain invisible to most humans.
"That's just how souls work," Mayuri said simply. To him, it was basic physics.
"Come on," Nanatsuki said. "Let's go check your old hiding place."
After a short walk, they arrived at the site of the Quincy War.
"Footprints… humans have been here. Did another Quincy clan move in?" Mayuri frowned.
The dirt road through the woods bore fresh tracks — proof of activity.
"No," Nanatsuki replied. "The Onmitsukidō guards withdrew two years ago. Their final report mentioned human settlers moving in."
The land was too fertile to remain empty — after the Quincy were wiped out, humans had inevitably reclaimed it.
"I hope my samples are still there," Mayuri muttered, uneasy.
"Didn't you say your hiding spots were foolproof?" Nanatsuki teased.
"I said they were hidden from Shinigami, not from humans," Mayuri retorted dryly.
A few minutes later, they reached a cornfield.
It was lush and well-maintained — clear evidence of human farming.
"Looks like they dug it up," Mayuri groaned. "I couldn't use Reishi barriers without alerting the Onmitsukidō, so I buried everything manually."
He sighed — his logic was sound, but reality was cruel.
"I'll check it myself."
He entered the cornfield and searched.
Moments later, he returned empty-handed.
"All gone?" Nanatsuki asked knowingly.
"All gone," Mayuri hissed. "Humans dug it up!"
"Let's head to the village," he said, still fuming. "Maybe the items ended up there."
"Sure," Nanatsuki smiled. "It'll be my first time visiting a human village — as a human myself."