Once a student receives their zanpakutō, the next step is to learn the method of soul infusion — channeling one's spiritual energy (reiatsu) into the blade. By doing this, a Shinigami can communicate with their zanpakutō's spirit, earning its recognition. Only after learning its true name (shikai no na) can they call it out to perform the Shikai release.
Of course, achieving Shikai is no easy feat — many graduates never accomplish it. This does not prevent them from serving as Shinigami, but those without release forms are far more limited in combat ability.
After obtaining the zanpakutō, students are taught soul infusion alongside the rest of their coursework.
In the fourth year, the academic subjects become far more specialized, including the theory of Senkaimon gate usage, the role and operation of Hell Butterflies (Jigokuchō), and detailed introductions to the Gotei 13 Divisions.
In addition to this, there are formal lessons in the theory of hakuda (hand-to-hand combat) and hoho (movement techniques), as well as an introduction to the principles of kidō.
By this stage, everything is oriented toward preparing students for actual combat duty. From first to third year, most subjects are purely theoretical and rarely applicable in real battles. But here in the fourth year, the training becomes practical — as it must for those who aim to wear the black kosode of the Shinigami.
Kurotsuki Renya knew these subjects were vital and approached them with complete focus. But among the practical training classes, it was Kidō and Shunpo that drew his keenest interest.
His first practical session was Kidō class. The instructor was Tsuno Haru, a seated officer from the Kidō Corps (Kidōshū). Though the previous Chief of Kidō and Vice-Chief had been forced into exile over a century ago, the Kidō Corps had since selected new leaders.
Instead of taking the class to the training yard, Tsuno Haru began in the lecture room.
"Kidō," he began, "is the high art of spiritual spellcraft — shaping reiryoku into techniques of offense, defense, and healing. It is divided into three main branches: Hadō, Bakudō, and Kaido."
He wrote the characters for each on the board as he explained:
"Hadō, or 'Way of Destruction,' is used for direct offense — turning spirit energy into flame, lightning, wind, or even constructs such as dragons. These are the battle spells of the Shinigami."
"Bakudō, the 'Way of Binding,' is used to restrain or defend. These techniques can bind an opponent with ropes of energy, seal their movements with light pillars, obscure vision with smoke, or conceal presence entirely. Though they cause little damage, their tactical value is immense."
Renya carefully noted every detail. The synergy of combining different techniques — much like Ishida Uryū's use of Quincy tools or Urahara Kisuke's unorthodox tactics — could turn the tide of a battle.
"Finally," Tsuno Haru continued, "Kaido — also called the Way of Healing — is restorative Kidō. However, Kaido requires a specific aptitude. Even the simplest healing spells are beyond the reach of most Shinigami. Those with the talent for it often join the 4th Division or the Kidō Corps for specialized training. We will not study Kaido until the second half of the year. For now, we will focus on Hadō and Bakudō."
He tapped the board for emphasis.
"These spells require incantations. Skilled practitioners can abandon the chant, but doing so significantly weakens the spell. Kidō are ranked from 1 to 99 — lower numbers are basic spells, higher numbers are vastly more destructive or complex. Spells from 1–30 are low-level, 31–60 are mid-level, 61–90 are high-level, and 91–99 are forbidden techniques — so powerful that even captains rarely master them."
"Low-level spells require the reiatsu of at least a Fourth Rank spiritual class. Mid-level spells need Fifth Rank. High-level require Sixth Rank, and forbidden Kidō… well, only a handful of individuals in recorded history have mastered them."
Renya thought immediately of the legendary Kidō masters — Yamamoto Genryūsai Shigekuni, Aizen Sōsuke, Kyōraku Shunsui, Urahara Kisuke, and Tessai Tsukabishi. Each was capable of casting high-level or forbidden Kidō with terrifying skill.
He knew mastery of such arts was a long-term goal. Even low and mid-level spells, if applied cleverly, could be decisive in combat. And unlike zanjutsu or hakuda, Kidō training also steadily strengthened spiritual pressure by consuming and replenishing reiryoku.
Renya finished his notes when a voice spoke beside him.
"Think all that's really useful?"
Looking over, he saw a tall, broad-shouldered classmate peering at his notebook. It was Shindō Ryōji, the same upperclassman who had spoken to him on his first day in the Fourth-Year class.
Renya adjusted his glasses slightly.
"I'd say it's more than useful."
Ryōji grinned.
"Then let me borrow your notes. I'll buy you lunch for the trouble."
Renya shook his head.
"Just take them. No need to pay me back."
Ryōji laughed.
"Good, because I'm broke until next month anyway."
Renya's brow twitched at the blunt honesty, but he knew Ryōji's easygoing manner hid a genuinely kind heart. Many students at the Shin'ō Academy were nobles, but just as many came from the outer districts of Rukongai. Ryōji was clearly the latter.
When class ended, Renya passed him the notebook.
"I'll give it back after afternoon lessons," Ryōji said with a smile.
Renya nodded and headed out of the classroom. Down the steps of the teaching building, he spotted Abarame Itsuki, Kuchiki Rukia, Jirō Yihe, and a young Kotetsu Isane waiting for him.
Even though they weren't in the same year, they often waited to eat together.
Renya smiled as he approached.
"Let's go."