In the depths of the underground laboratory, the lights flickered dimly.
"...Still a failure, huh."
It was Kurotsuchi Mayuri's first attempt at creating an artificial Shinigami. Rushed experiments could only lead to predictable results.
"You tried fusing a Gigai and an Artificial Soul to create a real Shinigami?"
Shuuichi's voice rang out from behind.
Mayuri turned to face him, meeting the comment head-on. "You figured that out?"
It frustrated him that Higashi Shuuichi, someone far less versed in science, wasn't the least bit interested in ending his life—and worse, had immediately dissected his intent.
"You rushed it," Shuuichi said bluntly, stepping past Mayuri toward the ruined experiment table. He gently lifted the failed Gigai.
"This was only because the trial was too rushed," Mayuri muttered, defensive. "If I had more time—"
"You think I'm critiquing this specific experiment, Mayuri?"
Shuuichi's eyes twinkled—he saw it. Mayuri was hooked. Unlike Urahara Kisuke's seamless mental defenses, Mayuri was just vulnerable enough to be manipulated.
"If not this experiment, then what—?" Mayuri asked, momentarily forgetting the entire reason Shuuichi had come.
"Gigai, Gikon... all of it is artificial. You tried to jump straight to fusing those artificial creations into true life. That leap is too wide. This isn't just difficult—it's absurd."
He held the failed Gigai between them like a mirror, hammering in the point.
"So what's the gentler path?" Mayuri frowned, sensing an implied solution.
"I'm no scientist, Mayuri. But during my time in the World of the Living, I learned two words from human researchers: technological foundation."
"Foundation…"
Mayuri's brow furrowed. But his gears were turning, exactly as Shuuichi had planned.
"Yes. What you're struggling with isn't the idea—it's the lack of foundation. You're like a baby trying to sprint before learning how to crawl. Sure, you might succeed eventually—but the effort will be hell, and full of dead ends."
"A crawling baby, huh…" Mayuri muttered, reflecting on his motivations. He had jumped into this pursuit of creating a new kind of being—mainly to prove he could surpass Urahara. It was ambition, not understanding, that had driven him here.
"Good metaphor," he conceded. "Which means… you have a better path in mind?"
Shuuichi grinned. Mayuri wasn't stupid—just arrogant. But that arrogance made him predictable.
"Have you heard of the humans in the Living World who call themselves Fullbringers?"
Mayuri nodded slowly. "Recently, yes."
"…Recently?"
That startled Shuuichi. He'd expected to need to explain the entire concept. Most Shinigami—especially those not assigned to Earth patrol—barely knew of Fullbringers.
But Mayuri was already aware?
"The armored warriors you fought on sub-level three? I retrieved them myself. From a Fullbringer."
A pause.
"Per orders from the Tsunayashiro Clan."
"So the leaked Gigai tech came from you too?" Shuuichi asked.
Mayuri raised an eyebrow. "You encountered more of them, I take it?"
He recalled how only someone like Urahara could deduce the Gigai fragments hidden in those constructs.
"The Tsunayashiro requested it. I simply followed orders—transferred archived schematics. The one who received the files… you know him."
"…I do?" Shuuichi frowned. Few people had both the clearance and capability to handle such tech. Only a handful came to mind: Urahara, Aizen, Mayuri himself, Yushima Ōshu… and one unassuming figure—Yamada Kiyonosuke.
It couldn't be him, could it?
He was about to entertain that notion when Mayuri dropped a bombshell.
"Karp Holmes. Captain of the Research Division in the Western Branch. Your file says he nearly killed you during a mission in the Living World."
Shuuichi froze.
That name—long buried—flashed back like a blade in the dark.
That man had nearly killed him. It was that defeat that made Shuuichi dive into forbidden realms, flirt with Hell itself… It all started with Karp.
And now it all made sense. The threads clicked into place: the West Division, the Fullbringers, the Tsunayashiro Clan, Aizen's suspicions…
The Kasaragi rebellion had inspired the great noble families to begin preparing private armies—forces that answered to no one but themselves.
The Tsunayashiro Clan clearly aimed to mimic the Kasaragi's plan. Not with Shinigami, though—they went to the Living World and started building an army of Fullbringers. Using stolen and forbidden tech. Partnering with the West Branch. All to create an independent war force.
But something still nagged at Shuuichi.
"The Tsunayashiro must know about Fullbringers' fatal weaknesses. They're human—short-lived. And because they carry fragments of the Soul King, they can't be purified via Konsō. If they die in the Living World, their souls have to cross the Dangai alone. That's nearly impossible."
(Author's note: this is an added rule to explain why Soul Society isn't flooded with Fullbringers, given later canon.)
So why invest in Fullbringers at all?
Perhaps the Tsunayashiro had other plans. Perhaps this was all a misdirection.
Back to the point.
"What do Fullbringers have to do with your artificial Shinigami?" Mayuri asked. He had no interest in the Tsunayashiro's agenda. He just wanted new toys.
Shuuichi grinned. "You've forgotten why I had to flee Soul Society with Urahara, haven't you?"
Then he released his Reiatsu.
A dark, corrosive pressure filled the chamber—violent, twisted.
Hollowfication.
A gray skeletal mask materialized on Shuuichi's face, his robes bleaching white.
"You've… mastered Hollow powers?!"
Mayuri's mind clicked instantly.
"No… wait… not just that," Shuuichi said, voice echoing beneath the mask. "The real question is—how did I become a Hollow?"
Understanding dawned on Mayuri.
"A true Shinigami should have not just Shinigami Reiatsu and a Shinigami soul—but Hollow essence as well."
"Exactly. So now you see, don't you? Mayuri—you could use Gigai, human souls, and Hollow energy, just like Fullbringer birth conditions. Recreate that triad. That's how you can test modified soul integration."
"And once that process is stable… I just swap the human soul out for an Artificial Soul, and boom—a real, functioning Shinigami."
Mayuri's eyes gleamed.
He didn't need more convincing.
"And what do you want from me, then?" he asked calmly. He knew this wasn't charity. But even so—he felt like he owed Shuuichi a huge favor already.
"Wait a sec," Shuuichi said, turning back to the failed Gigai. "I want to test something."
"Bankai—Resurrected Vessel."
His Reiatsu surged again. He poured it into the Gigai. Slowly, the dummy shimmered to life. He infused half his Reiatsu—enough to animate it—but its spiritual power barely rose past Lieutenant-class.
"…Just as I thought."
Shuuichi had suspected for a while that his Bankai, which allowed him to raise puppets from raw materials, had a limit. The artificial bodies couldn't channel the full extent of his power.
Even this upgraded Gigai, while having far better stamina, still couldn't output more than a basic Captain-level Reiatsu. And even that dropped quickly during combat.
Worse, his other Bankai—Hyakki Yagyō, which summoned demons from Hell—required physical bodies to anchor them. His spirit-clones were too ephemeral. The Gigai worked far better.
"Maybe one day I'll have a true Hundred Demon Parade," he mused. "Thousands of ghosts. All mine."
For now, he was stuck with fodder-tier ghosts—but that didn't stop him from fantasizing.
He turned to Mayuri, holding up three fingers.
"Three terms."
One: "I'll deliver Fullbringers to you personally. But I need you to grant me full operational freedom inside Soul Society."
Two: "I need as many Gigai as you can produce. And the Soul King fragments you extract from those Fullbringers—those are mine."
Three: "My apprentice, Yushima Ōshu, is currently a Twelfth Division grunt. I want him involved in the project."
"…That's it?"
Mayuri raised an eyebrow.
None of these were especially difficult.
The first—granting Shuuichi leeway—was doable as long as he didn't cause chaos.
The second—supplying Gigai—was child's play.
The third… was the only sticking point. Mayuri hated letting anyone into his lab. Especially someone who had once worked with Urahara.
"I'll allow it. But if Yushima doesn't impress me, I'll throw him out."
"Fair enough," Shuuichi agreed instantly. He had total confidence in Yushima's potential.
They discussed logistics for a while—how to smuggle Fullbringers in, how to communicate.
Finally, Shuuichi had Mayuri create him a new Gigai identical to the previous one and looted a few of his tech prototypes before casually walking out of the wrecked lab, smiling like a man who had just scored the deal of a lifetime.