The Mod Soul  

"Though I hate to admit it, Zaraki's reiatsu has completely surpassed mine…" Sensing the energy radiating from afar, Shunsui Kyōraku sighed.

As Head-Captain Yamamoto's most prized student, Shunsui had never disappointed his mentor, excelling in strength, temperament, intellect, and leadership. Yamamoto even saw him as his successor, grooming him to become the next Captain-Commander of the Gotei 13. Thus, many of the Seireitei's secrets were shared with Shunsui.

Decades ago, when Retsu Unohana had requested Yamamoto to teach Zaraki swordsmanship, the Head-Captain had agreed. However, upon meeting Zaraki, Yamamoto had changed his mind, teaching him a crude, unorthodox style that Zaraki despised, hoping to deter him from pursuing the path of the sword.

Yamamoto hadn't kept this from Shunsui. When questioned about his reasoning, Shunsui had witnessed, for the first time, fear in his teacher's eyes, the man known as the strongest Shinigami in a millennium. Even Captain Ailin hadn't elicited such a reaction.

Could it be that, in Yamamoto's eyes, Zaraki was an even greater threat than Ailin?

It was then that Shunsui had learned a shocking secret from his mentor.

"Sigh… This is troubling… If the mastermind lurking within the Seireitei is truly Ailin, then when he reveals his true intentions, on which side will Zaraki stand?"

The distant rumble of battle echoed through the air as Shunsui spread his reiatsu as far as he could, attempting to glean insights into the fight between Ailin and Zaraki. But as in previous decades, he could only sense Zaraki's reiatsu. Ailin remained hidden.

"Wait… That reiatsu…"

Shunsui suddenly shot up from his seat, his gaze fixed on the distant patch of golden clouds. He had just sensed a flicker of Ailin's reiatsu. It was faint, yet… it felt heavy.

Shunsui was no ordinary captain. For decades, driven by the pressure of Ailin and Sōsuke Aizen, he had trained relentlessly, his strength growing exponentially. His reiatsu was already approaching the limits for a captain. Yet, he could sense a "heaviness" in Ailin's reiatsu…

Such a level of power… Did even Old Man Yamamoto possess such strength? Had Ailin's reiatsu truly reached such a realm?

Moments later, Shunsui slumped back into his chair, a sense of dread washing over him. Zaraki's reiatsu had vanished.

No, not vanished. It had been suppressed in an instant, reduced to almost nothing. If he hadn't been paying close attention, he might have assumed that Zaraki had been killed.

Shunsui's eyes glazed over as the implications settled in. Zaraki, even before fully awakening, was a formidable opponent, his strength rivaling Shunsui's own. In a direct confrontation, Shunsui believed he had a 60% chance of victory. Initially, his Zanpakutō's abilities would give him the edge.

But if the battle descended into a chaotic brawl, the outcome was uncertain. Perhaps it would be a 50/50 split. Or maybe he would still maintain his advantage, though that 60%…

But even that Zaraki had been swiftly defeated. Judging by the brief flicker of Ailin's reiatsu, their captain was on a different level entirely.

…

After the battle between Zaraki and Ailin, life in the Seireitei seemingly returned to normal. However, beneath the surface, a sense of unease lingered. Almost all the captains intensified their training, pushing themselves to their limits.

What Shunsui had sensed, they had sensed it too, to a lesser degree. Faced with such overwhelming power, how much of their strength would they be able to utilize?

The members of the 11th Division also redoubled their efforts, training with unprecedented fervor. Kiganjō, Yachiru, and Ikkaku's first-hand account had driven home a stark reality. The next ten-year battle might be the most spectacular yet. If they didn't increase their strength, they might not even be qualified to watch.

A clash between Captain Ailin and Captain Zaraki would undoubtedly be a battle between the Seireitei's most elite fighters. To miss such a spectacle would be a lifelong regret.

…

Unlike the members of the 11th Division, Rukia Kuchiki's duties within the 13th Division were incredibly light. However, her seemingly ordinary performance fueled resentment among those who believed she had only avoided being sent to the Hollow extermination forces because of her noble status.

They argued that her presence was an insult to the students who had failed the exam and been sent to Hueco Mundo. This was the unfair system that favored nobles. Rukia, in their eyes, was an incompetent noble, relying on her family connections to secure a position within the Gotei 13.

As a result, she faced constant resistance and harassment. Her tasks were deliberately made more difficult and time-consuming. Her fellow Shinigami refused to cooperate, forcing her to complete her duties alone.

Rukia, however, chose to endure it. She hadn't retaliated. As a result, no one came to her defense. After all, she hadn't displayed any strength that warranted their respect.

It wasn't until Kaien Shiba, the lieutenant of the 13th Division, discovered her surname and, out of respect for the bond between the Four Great Noble Clans, extended a hand of friendship. He treated Rukia with kindness, creating a brief period of happiness within the division.

…

Within the Shinigami Research and Development Institute of the 12th Division, a man with half-green, half-white hair sat before a wall of glowing screens, his fingers flying across the keyboard. Connected to the computer were dozens of incubation tanks filled with green nutrient solutions. On a circular platform, a small test tube held half a vial of swirling blue reishi.

This was Kageroza Inaba, the seventh seat of the 12th Division and one of the institute's most brilliant researchers. His knowledge of the Dangai, the dimensional void between worlds, was unmatched within the division. As a result, he held the title of "Director of Dangai Research for the Seireitei," controlling the access of countless Shinigami to the World of the Living.

However, this was only one side of Kageroza Inaba. He wasn't a true Shinigami. In fact, he wasn't even human.

Centuries ago, a Shin'ō Academy student named Ōko Yushima had barely passed his graduation exam, eventually joining the 9th Division. Unfortunately, his lack of strength made him a target for bullying. Eventually, he was expelled and sent to the 12th Division, a division that didn't require exceptional combat skills.

It was within the 12th Division that Ōko's research talents were fully unlocked by Kirio Hikifune, the former captain. He even surpassed his mentor in the field of soul modification. However, Hikifune's research had focused on perfecting her "Soul Implantation" technique. Once she had successfully created modified souls, she had moved on, focusing on developing a method to implant those "temporary souls" into Shinigami, boosting their reiatsu.

Ōko, however, continued his research in soul modification, eventually creating controllable "Mod Souls," artificial souls that could be implanted into inanimate objects, granting them a semblance of life and consciousness. He used the reishi of Shinigami to create these modified souls, imbuing them with the memories and abilities of the deceased.

This meant that as long as the Seireitei provided him with the reishi of powerful Shinigami, he could create an army of Mod Souls. Sadly, when he presented his findings and proposed his plan to the Central 46, the nobles, fearing that Ōko would use this army to overthrow their rule, rejected it. They banned his research and sentenced him to imprisonment in Muken, the inescapable prison for those who dared to challenge their authority.

Disillusioned with the Soul Society, and to evade capture while overcoming his own lack of strength, Ōko used Hikifune's "Soul Implantation" research to split himself into two beings. Once they reached a certain level of power, they would merge, their reiatsu amplified by the complete "Soul Implantation" process.