"Oh? Captain Unohana comes to teach at the Academy too?"
Hao was a little surprised when he heard that.
Tessai Tsukabishi nodded. "Of course. Many captains from the Gotei 13 come to teach here each year for a short period. Since Squad Four needs to recruit Shinigami skilled in Kaidō, Captain Unohana often comes to give lectures—part of a process to screen for future Squad Four members."
He glanced at the students practicing Hadō in the open field and continued, "Actually, my class focuses mainly on Hadō. I also share some insights on Bakudō, but Kaidō is a separate elective course. It's usually taught by seated officers, lieutenants, or Captain Unohana herself."
Hao understood after hearing that. Kaidō required extremely delicate control, and not many Shinigami had a natural talent for it. Most of the young people joining Shin'ō Academy came with ambition—they preferred combat and fighting, not support work.
As a result, Squad Four always had trouble recruiting members. Kaidō being offered as a standalone elective meant that those who did well in it would likely end up joining Squad Four in the future.
After class, Hao and Aizen had a satisfying meal at the cafeteria and returned to their dorm.
"Sōsuke, if I want to take an elective here at the Academy, do I need to apply in advance?" Hao asked with curiosity. Although he had transferred in midway through the term, if the Academy offered electives, he figured they should at least give him a course list to choose from.
"No need for that," Aizen replied. "Shin'ō Academy offers a wide range of electives. You can just attend any class you like. But if you want to get credit, you'll have to pass the examiner's test at the end of the semester. Your results will be recorded in your file, and when the squads pick new recruits later on, they mostly review those files."
He added with a smile, "Why? Are you thinking about taking the Kaidō class, Hao-kun?"
Hao nodded. "I'm a bit interested in Kaidō."
"To enjoy battle more fully, huh? You're really quite the fighter."
Aizen was starting to get a better sense of Hao. Like himself, Hao was a genius—but unlike Aizen, he didn't seem to have any deep schemes. Just a straightforward, battle-hungry warrior.
Not that Aizen disliked that kind of person. On the contrary, having a genius roommate like Hao actually added some much-needed amusement to his otherwise boring life.
"Don't you think combat is fun too, Sōsuke? I get the feeling you'd make a great opponent."
Hao grinned. Aizen was still hiding his true strength, and on the surface, his spiritual pressure seemed on par with Hao's—both ranked at level 17. Aizen had claimed he started out at level 19 when he first enrolled, and his current power was the result of hard training.
But in truth, Hao suspected that Aizen's actual level was at least minus the '1' in front.
"You flatter me, Hao-kun. I'm not on your level. In swordsmanship and hand-to-hand combat, I'm not your match. Even in Kido, I don't have your talent."
Aizen said modestly, then smoothly changed the subject. "By the way, I noticed you already have your own Asauchi. You haven't taken any classes on Zanpakutō yet, right? I could teach you a few things early—basic knowledge about Zanpakutō and how to strengthen your spiritual pressure."
"Oh?"
Hao perked up. "Then I'll be counting on you, Sensei Sōsuke."
Aizen chuckled. "No need to call me that—I only know a little. Think of it as some light preview work before the real lessons."
What followed was Aizen explaining the fundamentals of Zanpakutō and methods for cultivating spiritual pressure.
His goal was simple—he didn't dislike Hao. In fact, he quite liked this genius roommate of his. Building a good relationship certainly wouldn't hurt. And if Hao became stronger, he might even become useful for Aizen's future plans.
He was now also curious—was Hao truly an all-around genius like himself? Did he possess natural talent in Zanpakutō resonance and spiritual pressure as well?
Aizen was, indeed, a great teacher. Hao found that listening to Aizen's lectures made understanding the material much easier than the theoretical courses taught by the other instructors during the day.
First came the topic of spiritual pressure enhancement. Since Shinigami are composed of high-density spirit particles, increasing spiritual pressure essentially means increasing the density of those particles within oneself.
This could be done through good nutrition, absorbing ambient reishi and refining it into one's spirit body, and also through physical training.
Yes, training the body could improve spiritual pressure as well. A Shinigami's "physical strength" was essentially a measure of their soul's density. So even conventional body training counted as a valid method of strengthening spiritual power.
However, Aizen typically didn't rely on that method. He had exceptional control over spirit particles and would instead absorb the ones freely floating in the air, gather and integrate them, and thus enhance the density of his spirit body to raise his spiritual pressure.
Aizen's theory coincided with Hao's own speculations back when he read the manga. For example, in the Thousand-Year Blood War arc, Kirio Hikifune said that eating food infused with her spirit particles could increase one's reiryoku.
Likewise, Hollows—another kind of being in this world—strengthened themselves by devouring one another to evolve upward. That, too, was a form of reiryoku absorption.
From this, one could conclude that increasing spiritual pressure essentially comes down to improving the fusion capacity of reishi.
That's why Aizen's spiritual pressure was so overwhelming. Part of it was raw talent and an innately high cap—but the other part was how quickly he advanced through training.
Aside from regular training, there was another method that could cause an explosive increase in spiritual pressure: the release of a Zanpakutō.
However, it's important to note—not all Zanpakutō boosts their wielder's spiritual pressure upon release. In fact, most don't.
The enhanced spiritual pressure others feel during a Shikai or Bankai is often an illusion. The user's actual spiritual pressure doesn't increase—it's the output that changes.
To illustrate: suppose a Shinigami's spiritual pressure is 100. Before Shikai, their maximum output per attack might be 5. After releasing Shikai, that output might go up to 10. Upon reaching Bankai, it could spike to 40 or 50.
There are rare Zanpakutō, though, that truly do increase spiritual pressure upon release. For example, Ichigo Kurosaki's blade significantly boosted his reiryoku during Bankai. Or Kenpachi Zaraki—his opponents clearly sensed his spiritual pressure rising when he unleashed his Bankai.
Still, Zanpakutō that grant a genuine increase in reiryoku after release are rare, and the boost is usually minimal. Ichigo was a protagonist-tier exception.
All in all, though, a Shinigami who has mastered their Zanpakutō's release is on an entirely different power level from one who hasn't.
So if Hao wanted to dramatically increase his strength, learning to release his Zanpakutō was the most effective route.
But there was a catch—learning Shikai wasn't easy. First, one needed a baseline level of spiritual power. A level 17 in spiritual pressure barely reached the entry threshold for Shikai.
Typically, only seated officers within the Gotei 13 could achieve it. They were usually at least level 10 in reiryoku.
According to Aizen, the higher one's spiritual pressure ranking, the easier it was to resonate with one's Zanpakutō. That's because the level reflected how well they had tapped into their soul's power. After all, the Shikai release of a Zanpakutō was an embodiment of a Shinigami's soul.
To put it plainly, an Asauchi is just a blank blade. Only when a Shinigami infuses it with their soul does it become a true Zanpakutō.
The abilities of a Zanpakutō are influenced by the soul of its wielder. To an extreme degree, you could even say a Zanpakutō is the outward manifestation of a Shinigami's soul.
"Sōsuke, you're the top student here—have you learned Shikai yet?"
Hao asked out of curiosity, mainly to see how Aizen would respond.
To Hao's surprise, the usually reserved Aizen actually nodded and replied with a smile, "My Zanpakutō is called Kyōka Suigetsu. It's a water-type blade. By reflecting light through mist and water flow, it confuses enemies and causes them to attack each other."
Sitting cross-legged on the bed, Aizen gently touched the Kyōka Suigetsu laid out in front of his knees. Then he looked at Hao.
"Hao-kun, would you like to see my Zanpakutō?"
°°°
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