Hikifune Kirio had advanced the "Temporary Soul" technique ahead of schedule, and she was very likely to be promoted to the Royal Guard sooner than expected.
That meant the position of Captain of the 12th Division might be vacant for a while.
Nanatsuki's mind immediately became active.
There were only two potential candidates who could compete for the 12th Division captaincy in the future: Urahara Kisuke and Mayuri Kurotsuchi.
Urahara Kisuke was still too young; it would take time for him to mature.
Nanatsuki was confident he could reach captain level before Urahara Kisuke.
As for the other research-type candidate, Mayuri Kurotsuchi, Nanatsuki had already heard his name a couple of weeks ago.
He had been discovered by the 2nd Division conducting secret research on Quincy, and was currently imprisoned in the Maggot's Nest.
Under normal circumstances, he would only be released after Urahara Kisuke became a captain.
So Nanatsuki only needed to raise his strength to the level of a captain before Urahara Kisuke did.
Then, he could secure the captain position of the 12th Division and inherit the laboratories left behind by Hikifune Kirio, Shutara Senjumaru, and others.
Nanatsuki now possessed the Reishi Shell technique, had a high level of Kaido proficiency, and extensive medical knowledge. This gave him deep insight into spirit-based bodies.
He already had the foundation needed for spiritual body research.
If he could inherit the laboratories left behind by Hikifune Kirio and Shutara Senjumaru, it would greatly aid his work in spirit body studies.
The research resources available to a 12th Division captain were incomparable to what ordinary people had access to.
He could even follow Urahara Kisuke's example by turning the 12th Division into a Technology Development Bureau and recruiting researchers to assist him.
Moreover, the Katori-ryu Dojo's reputation had already spread—recruiting new disciples was no longer a problem.
Even if he stepped down from his position as a Shin'o Spiritual Arts Academy instructor, he would still attract students.
Just like Unohana Retsu, he could give swordsmanship lectures at the Academy a few times each semester, while scouting for new talent.
"Captain, Soul #69 is beginning to dissipate," Urahara Kisuke called out to Hikifune Kirio.
Katori Nanatsuki followed Hikifune Kirio to stand before a transparent glass tank.
The tank was filled with a high-concentration Reishi solution, and within it floated an unidentified mass.
At first glance, the mass resembled a bizarre, tumor-like growth.
But upon closer inspection, one would see that it was composed of hundreds—perhaps thousands—of small nodules condensed into one.
"This is an early experimental subject," Hikifune Kirio explained to Nanatsuki. "We tried to use Reishi clusters to assemble a rodent-type spirit body—to construct organs and body parts from the clusters."
"Unfortunately, it didn't meet expectations. These clusters condensed together, but no self-awareness emerged. They just became a larger 'organ.'"
"No consciousness formed?" Nanatsuki's mind stirred. "Then could this technique be used to cultivate organs?"
"By growing organs that are compatible with a host, we might heal wounds that Kaido can't cure. In such cases, there's no need to produce a conscious soul... Like the Spiritual Pressure Core."
"Some cultivated organs have even been successfully transplanted into Shinigami." Hikifune Kirio let out a small sigh and added, "You're trying to heal Batsu'unsai's Spiritual Pressure Core, aren't you?"
"To use this technique to grow a Spiritual Pressure Core for Batsu'unsai, there are still two major problems."
"You can see for yourself—these Reishi clusters are beginning to break down."
"They lack the ability to absorb Reishi from the environment. In fact, their own Reishi continues to leak outward. As a result..."
"...the Reishi inside the clusters steadily diminishes. Once it drops below a certain threshold, the structure collapses."
"So these creations have very short lifespans—they're definitely unsuitable for making a Spiritual Pressure Core."
Nanatsuki nodded. After watching for a while, he could also tell the growth had entered a decaying state. It was disintegrating and fading away.
"But thanks to you, Nanatsuki-kun, I was inspired by the Reiatsu-Converging Pill and managed to improve the structure of the Reishi membrane, granting it the ability to absorb Reishi," Hikifune Kirio continued.
She then led Nanatsuki to another glass tank.
This tank also held a cluster submerged in high-concentration Reishi solution.
Hikifune Kirio smiled. "The experimental results are promising. The improved membrane not only allows the cluster to absorb Reishi, but also slows down the rate of dissipation."
"Look—it's still 'alive,' though it lacks self-awareness."
This Reishi cluster roughly resembled a mouse.
"Then is this technique ready for cultivating a Spiritual Pressure Core?" Nanatsuki asked.
"It can cultivate a Spiritual Pressure Core, but not one strong enough for Batsu'unsai," Hikifune Kirio shook her head. "There's still one more issue."
"If we connect the core we cultivate to Batsu'unsai's Lock and Soul Sleep, her body would regain the ability to produce Reiatsu on its own."
"But her Reiatsu is far too strong. The fragile core would be instantly crushed by it."
"The souls I cultivate don't have growth potential. The Spiritual Pressure Cores produced aren't durable enough to support her Reiatsu."
Nanatsuki nodded in understanding—this was the core technical challenge.
The Spiritual Pressure Core cultivated by Hikifune Kirio's technique was akin to a child's heart—completely incapable of sustaining an adult's body.
To help Batsu'unsai, the issue of soul growth needed to be solved. Only a core that could withstand her overwhelming Reiatsu could be viable.
Nanatsuki sensed someone watching him. He looked up and saw Urahara Kisuke observing him.
"So you're Katori Nanatsuki? Your Reiatsu-Converging Pill technique was truly innovative. It's sped up the Captain's research by a lot," Urahara said with a calm smile. "Captain, aren't you going to recruit him?"
Urahara Kisuke had clearly overheard Nanatsuki and Hikifune's conversation—and realized Nanatsuki had a strong interest in their research.
"Ara ara, I'd love to invite Nanatsuki-kun to the lab, but he seems to prefer teaching at the Academy," Hikifune Kirio replied, shaking her head with a helpless smile.
"Captain Hikifune, I just got promoted recently, you know. At least let me enjoy a few more days as Head of the Swordsmanship Department," Nanatsuki joked. "But if I ever leave the Academy, the 12th Division will be my first choice."
Nanatsuki hadn't spoken in absolutes—he was seriously considering joining the 12th Division. But what he had in mind was the position of captain.
"Hahaha—didn't you say the same thing to Unohana? You're such a sly little fox!" Hikifune Kirio laughed heartily.
"It's not the same," Nanatsuki replied. "After all, the 4th Division doesn't serve Captain Hikifune's delicious food!"