"Oh? Captain Unohana even comes to the Academy to teach classes?"
Kurosaki Rei was a little surprised when he heard this.
Tessai Tsukabishi nodded. "Of course. Quite a few captains from the Gotei Thirteen come to teach for a period every year. And since the Fourth Division needs to recruit Shinigami who are proficient in Kidō healing techniques, Captain Unohana often comes to teach as well—basically to screen future Fourth Division members."
He glanced at the students on the open ground practicing Hadō and continued, "Strictly speaking, the classes I teach mainly focus on Hadō. I also share some insights on Bakudō. As for Kaidō, it's been separated into an independent elective. Most of the time, it's taught by seated officers, vice-captains of the Fourth Division, or Captain Unohana herself."
After listening, Kurosaki Rei understood. Kaidō was precision work of the highest level. Shinigami with talent in that area were truly rare. Moreover, most young people who entered the Shin'ō Academy were ambitious and preferred fighting and combat rather than doing support work.
As a result, recruiting for the Fourth Division had always been a problem. Kaidō was offered as a separate elective, and those who chose it and achieved good results would, in most cases, join the Fourth Division in the future.
After class, Kurosaki Rei and Aizen returned to the dormitory after enjoying a satisfying meal in the cafeteria.
"Sōsuke, if we want to attend elective courses at the Academy, do we need to apply in advance?" Kurosaki Rei asked in confusion. Although he had transferred in midway, since the Academy offered electives, he should at least have been given a course schedule to choose from.
"That won't be necessary," Aizen explained. "The Shin'ō Academy has many elective courses. You can just go and listen. But if you want to earn a grade, you'll need to pass the examiner's test during the final exams. Only then will it be recorded in your file. When the divisions select new members in the future, they usually choose people by reviewing those files."
He then asked, "Why? Are you thinking of taking the Kaidō elective, Rei-kun?"
Kurosaki Rei nodded. "I'm a bit interested in Kaidō."
"For the sake of enjoying battle more?" Aizen smiled faintly. "Rei-kun, you really are battle-hungry."
Aizen felt he was starting to understand Kurosaki Rei. Like himself, Kurosaki Rei was a once-in-a-generation genius, but unlike Aizen, he had no deep or scheming thoughts—just a straightforward, battle-loving brute.
Aizen didn't dislike that type of person. On the contrary, having a fellow genius as a roommate made his otherwise boring life much more interesting.
"Sōsuke, don't you think fighting is fun?" Kurosaki Rei asked with a grin. "I always feel like you'd make a great opponent."
Aizen was currently hiding his true strength. On the surface, his spiritual pressure seemed similar to Kurosaki Rei's, both at Level 17. According to Aizen himself, he had been Level 19 when he first enrolled, and his current level was the result of diligent training.
In reality, Kurosaki Rei felt that Aizen's true spiritual pressure level had probably removed the leading "1" entirely.
"Rei-kun jokes too much," Aizen said calmly. "Whether it's swordsmanship or Hakuda, I'm no match for you. And in Kidō, my talent isn't as good as yours either."
He smoothly changed the subject. "That said, I noticed you already have your own Asauchi. Although you haven't taken any classes on Zanpakutō yet, I can teach you some basic knowledge about Zanpakutō and ways to improve spiritual pressure in advance."
"Oh?" Kurosaki Rei perked up. "Then I'll be in your care, Teacher Sōsuke."
Aizen chuckled. "You don't need to call me that. I only know a little about these things—just enough to give you some pre-class preparation."
Next, Aizen explained the basic knowledge of Zanpakutō and methods of spiritual pressure cultivation.
Aizen's goal was simple. He didn't dislike Kurosaki Rei—in fact, he quite liked his genius roommate. Building a good relationship wouldn't hurt, and making Kurosaki Rei stronger might even help with his future plans.
Besides, Aizen also wanted to know whether Kurosaki Rei was, like himself, a well-rounded genius—whether he had talent in Zanpakutō resonance and in the realm of spiritual pressure.
Aizen was indeed a good teacher. Kurosaki Rei found that listening to Aizen's explanations was far easier to understand than the theoretical lectures from several instructors during the day.
First came the improvement of spiritual pressure. Since Shinigami were composed of high-density Reishi, increasing spiritual pressure essentially meant increasing one's Reishi density.
This could be achieved through high-quality food, absorbing external Reishi and compressing or refining it into one's spiritual body, as well as physical training.
Yes—training the body could also increase spiritual pressure. The strength of a Shinigami's "body" was, in essence, an increase in soul density. So even conventional physical training was an effective way to improve spiritual pressure.
However, Aizen usually didn't use that method. He had extremely strong control over Reishi, so he absorbed the free Reishi in the air, consolidated it, and refined it into his own spiritual body, thereby increasing his spiritual pressure.
This theory closely matched Kurosaki Rei's own speculation from when he had read the manga before. For example, in the Thousand-Year Blood War arc, Kirio Hikifune mentioned that eating food containing her Reishi could increase spiritual pressure.
Another creature in this world—the Hollow—followed a similar logic. By devouring their own kind and acquiring their spiritual pressure, Hollows evolved upward. They could increase their spiritual pressure through cannibalism.
Taken together, increasing spiritual pressure was fundamentally about improving the bonding capacity between Reishi particles.
That was why Aizen's spiritual pressure was so strong. On one hand, his natural talent gave him a high upper limit. On the other hand, his "training" speed was also extremely fast.
Aside from everyday cultivation methods, there was also a way to explosively increase combat capability—and that was Zanpakutō release.
One point had to be made clear: not all Zanpakutō releases increased the user's spiritual pressure. In fact, most Zanpakutō didn't have this function at all.
The sense of increased spiritual pressure an opponent felt after Shikai or Bankai was mostly an illusion. In essence, the user's spiritual pressure didn't increase; rather, their output efficiency changed.
For example, suppose a Shinigami had a total spiritual pressure of 100. Before Shikai, their maximum output per strike might be 5. After Shikai, that maximum output might become 10. With Bankai, it could reach 40–50.
Of course, there were a few Zanpakutō whose release truly did increase spiritual pressure to some extent—such as the protagonist Kurosaki Ichigo's Zanpakutō, whose Bankai clearly raised his spiritual pressure, or Zaraki Kenpachi's Zanpakutō, where opponents explicitly sensed his spiritual pressure surging after release.
Still, Zanpakutō that increased spiritual pressure upon release were rare, and the boost was usually not that large. Ichigo was a special case as the protagonist.
Overall, a Shinigami who had mastered Zanpakutō release and one who hadn't were on entirely different levels of combat power.
So if Kurosaki Rei wanted a significant boost in strength, mastering Zanpakutō release was the most effective route.
But there was a problem. Learning Shikai wasn't easy. First, one needed a certain baseline of spiritual pressure. Level 17 spiritual pressure was only just enough to touch the threshold of Shikai.
Usually, those who mastered Shikai were seated officers in the Gotei Thirteen. Such officers typically had spiritual pressure of at least Level 10.
According to Aizen, the higher one's spiritual pressure level, the easier it was to resonate with one's Zanpakutō. This indicated a deeper development of one's soul power. Shikai, in essence, was a manifestation of a Shinigami's soul.
Simply put, an Asauchi was just an Asauchi. Only when one infused their own soul into it did it become a true Zanpakutō.
A Zanpakutō's ability would be influenced by the Shinigami's soul. Taken to an extreme, it could be understood as an externalized form of the Shinigami's soul.
"Sōsuke, you're one of the top students here. Have you already mastered Shikai?" Kurosaki Rei asked curiously, mainly wanting to see Aizen's reaction.
What surprised him was that Aizen—who usually concealed his strength—actually nodded this time. With a gentle smile, he said, "My Zanpakutō is named Kyōka Suigetsu. It's a water-flow type Zanpakutō, capable of disrupting the enemy through the chaotic refraction of mist and flowing water, causing them to kill each other."
Sitting cross-legged on the bed, he touched Kyōka Suigetsu resting on his knees, then looked at Kurosaki Rei.
"Rei-kun… would you like to take a look at my Zanpakutō?"