Chapter 13: Would You Like to See My Shikai?

"Oh? Captain Unohana also substitutes as a lecturer at the academy?" Ren asked, visibly surprised.

Tessai nodded. "Of course. Several captains from the Gotei 13 teach at the academy every year. Since the Fourth Division needs to recruit Shinigami skilled in Kaido, Captain Unohana often volunteers to lecture. It's also a way to identify potential candidates for the Fourth Division."

He glanced at the students practising destructive Kido on the field and continued, "I primarily teach classes focused on Hadō, though I also provide insights into Bakudō. Kaido, however, is offered as a standalone elective, typically taught by officers or the vice-captain of the Fourth Division—and sometimes Captain Unohana herself."

Hearing this, Ren finally understood. Kaido, as the most precise spiritual art, naturally attracted fewer students since its delicate nature required a rare aptitude.

Most recruits at Shin'o Academy were more drawn to the action-oriented aspects of Shinigami life—preferring swordplay and battle over supportive roles. This made recruitment for the Fourth Division a perennial challenge, which is why Kaido classes served as both instruction and screening for future candidates.

After class, Ren and Aizen returned to their dorm after a satisfying meal in the cafeteria.

"Sōsuke, do we need to apply in advance to take an elective course here?" Ren asked.

Although he had joined mid-term, he figured the academy should at least provide him with a course selection guide.

"There's no need for that," Aizen replied. "The academy offers many electives. You're free to attend any of them, but if you want the credits recorded in your file, you'll need to pass the final assessment given by the instructor. These grades are what squad captains review when selecting recruits."

"So, you are interested in....Kaido?" Aizen inquired.

Ren nodded. "Yeah, it's caught my interest."

"Is it for the sake of enhancing your enjoyment of combat? You're truly a battle enthusiast, Ren-kun," Aizen observed with a faint smile, feeling like he had begun to understand Ren. While they were both extraordinary geniuses, Ren lacked the subtle cunning that Aizen harboured. He was straightforward, bold, and utterly driven by his passion for fighting.

Aizen didn't dislike this. In fact, having such an exceptional and refreshingly direct roommate added a bit of colour to his otherwise monotonous life.

"Don't you find combat exhilarating, Sōsuke? I can't help but think you'd make an excellent sparring partner," Ren remarked with a grin.

Although Aizen masked his true strength, his spiritual pressure matched Ren's on paper, with both the roommates ranked at level 17.

Aizen had claimed that his current strength was the result of hard work, having improved from level 19 at the start of the semester.

However, Ren suspected that if Aizen's actual power were measured, the "1" in his rank could be discarded altogether.

"You flatter me, Ren-kun. I'm far from your level. In swordsmanship and hand-to-hand combat, I wouldn't stand a chance. Even in Kido, your natural talent surpasses mine." Aizen deflected the compliment, smoothly changing the subject. "By the way, I noticed you already have your Asauchi. Although you haven't taken any classes on Zanpakuto yet, I could give you a primer on them and some tips for improving your spiritual pressure."

"Oh?" Ren perked up, intrigued. "I'd appreciate it, Sōsuke-sensei."

Aizen chuckled softly. "Please, don't call me that. I only know a little more than you—think of it as a preview of future lessons."

What followed was a masterclass of sorts, with Aizen explaining the fundamentals of Zanpakuto and methods for cultivating spiritual pressure.

Aizen's motives were straightforward: he genuinely liked his talented roommate and saw no harm in forming a good rapport. Helping Ren grow stronger might even serve his future plans. At the very least, Aizen was curious to see whether Ren was a true all-rounder, capable of excelling even in fields like Zanpakuto resonance and spiritual pressure refinement.

Under Aizen's guidance, Ren found the lessons remarkably clear—more so than any of the day's formal classes.

First, Aizen outlined the basics of spiritual pressure enhancement. Since Shinigami are composed of highly condensed reishi (spiritual particles), increasing spiritual pressure meant increasing the density of these particles within one's soul.

This could be achieved through a proper diet enriched with reishi, absorbing and refining ambient reishi into one's soul or the plain-ol physical training, which also worked to strengthen the "spiritual body" since physical enhancement inherently increased soul density.

Interestingly, Aizen himself didn't rely on physical methods. His natural affinity for reishi manipulation was so strong that it allowed him to effortlessly absorb and refine the ambient reishi to bolster his spiritual density.

This explanation resonated with Ren. It aligned with ideas he'd speculated about before, like when Hikifune Kirio in the Thousand-Year Blood War arc revealed that consuming food imbued with her reishi could directly increase spiritual pressure.

Similarly, Hollows evolved by devouring each other to absorb spiritual energy.

In essence, increasing spiritual pressure boiled down to improving the fusion and manipulation of reishi.

Unsurprisingly, Aizen's spiritual pressure was extraordinarily high—not only because of his natural talent but also due to his unmatched speed of gathering and absorption of reishi.

Aizen also touched on explosive methods to temporarily amplify spiritual pressure:

the Shikai and Bankai of Zanpakuto.

It is important to clarify that not all Zanpakuto boosts their wielder's spiritual pressure upon release.

In fact, most did not.

The perceived increase in power was due to a change in output efficiency. For instance, a Shinigami with a baseline spiritual pressure of 100 might have a maximum single-strike output of 5. Upon achieving Shikai, this output might increase to 10. Bankai could raise it further, perhaps to 40 or 50.

There are, however, rare Zanpakuto that do genuinely increase the wielder's spiritual pressure upon release. For instance, Ichigo Kurosaki's Zanpakuto noticeably boosts his spiritual pressure when he achieves Bankai, and similarly, Kenpachi Zaraki's Zanpakuto clearly demonstrates a surge in spiritual pressure during his Bankai, as perceived by his opponents.

Still, Zanpakuto with such enhancements are the exception, and even then, the increase in spiritual pressure is usually modest. Ichigo, of course, is an exception to the norm as the protagonist.

That said, mastering a Zanpakuto's release—be it Shikai or Bankai—fundamentally elevates a Shinigami to an entirely different level of combat power.

That's why it is almost unanimously accepted that shinigami who have mastered the release of their Zanpakuto and those who have not are two completely different creatures.

Therefore, for Ren, achieving Zanpakuto's release would be the most direct path to significantly improving his strength.

However, there was a catch: learning Shikai wasn't easy. It required a foundational level of spiritual pressure, and Ren's current rank of 17 barely met the threshold.

Typically, those who mastered Shikai were seated officers within the Gotei 13, and their spiritual pressure levels were generally at least rank 10.

According to Aizen, the higher a Shinigami's spiritual pressure level, the easier it was to resonate with their Zanpakuto. This correlation implied that a deeper understanding and development of one's soul power directly influenced the ability to achieve Shikai.

Put simply, an Asauchi remained just that until a Shinigami infused it with their soul. Only then did it become a true Zanpakuto.

Moreover, the Zanpakuto's abilities were shaped by the wielder's soul. In extreme terms, one could view a Zanpakuto as a manifestation of a Shinigami's externalized soul.

"Sōsuke, you're one of the top students here. Have you mastered Shikai yet?" Ren asked with curiosity, mostly to gauge Aizen's reaction.

To his surprise, the usually reserved Aizen nodded this time, smiling as he replied, "Yes, I have. My Zanpakuto is named Kyoka Suigetsu. It's a water-type Zanpakuto that uses the refraction of mist and water to confuse enemies, causing them to attack each other."

Seated cross-legged on his bed, Aizen gently ran his fingers along the blade of Kyoka Suigetsu resting before him. Then he turned to Ren with an inviting smile.

"Would you like to see my Shikai, Ren-kun?"

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