Naruto: Aizen's Shadow in Konoha

Determined to cultivate his power, Aizen recognized the importance of mastering energy control. This was especially true for Kyoka Suigetsu, his most potent weapon at this stage.

The blade was a gluttonous consumer of energy. Aizen had already attempted practicing with Reiryoku, a far superior form of energy compared to regular chakra. However, whether he could utilize it in this world remained a mystery. In Soul Society, cultivating Reiryoku involved absorbing stray spirits that permeated the atmosphere.

But did such "spirit particles" even exist in the ninja world?

Despite the uncertainty, Aizen gave it a shot. As expected, the results were a negative.

However, a surprising turn of events emerged. While Reiryoku proved a dead end, Aizen discovered that the Shinigami methods for absorption and cultivation surprisingly worked on the ambient energy present in the world of ninjas.

This energy, if labeled, was none other than natural energy!

This discovery filled Aizen with elation. By combining his own physical energy with chakra, he could potentially create a potent form of chakra – Senjutsu chakra! This high-level energy form in the ninja world rivaled even the power of Kekkei Genkai.

Successfully cultivating the Senjutsu chakra would drastically enhance his strength and make Kyoka Suigetsu even more formidable. Nevertheless, a wave of caution washed over him before he could get too carried away. Though powerful, natural energy also comes with significant risks!

The original Naruto series depicted the dangers of natural energy cultivation. Mount Myoboku, a haven for toads who'd honed Senjutsu techniques for millennia, still had numerous petrified toads – a testament to the significant risks involved.

Furthermore, there was no guarantee that the Shinigami method of absorbing spirit particles would seamlessly translate to absorbing natural energy in the ninja world. While it appeared promising initially, Aizen, burdened by the memories of his past life, wasn't willing to take such a gamble.

This didn't mean abandoning his plan to cultivate Senjutsu chakra entirely. The ninja world offered a practice technique perfectly suited to mitigate these risks – the Shadow Clone Jutsu!

This signature jutsu of the series protagonist not only created a physical clone by dividing one's chakra reserves but also relayed all the clone's experiences and sensory information back to the main body – essentially a cheat code for training. Naruto Uzumaki himself utilized this technique to master Senjutsu.

While dispelling a shadow clone returned its chakra to the main body, it wasn't without its own risks. The core connection between the clone and the user remained the soul. And while targeting souls proved difficult in the ninja world, Shinigami possessed a method for severing this connection – the simplest being a clean cut with a Zanpakuto.

Training with shadow clones meant that even failure would only result in the loss of a chakra clone. However, there was an additional issue: upon severing the connection, the clone's bodily changes wouldn't be transferred back to the main body.

This posed a slight challenge. But Aizen, ever resourceful, devised a solution.

Taking a scientific approach, he planned to meticulously record his attempts at absorbing natural energy each time. He would then observe the resulting changes in the shadow clone's form and analyze the specific issues encountered. By systematically addressing these problems, he could pave the way for successful Senjutsu cultivation.

This method might seem stupid, but that's the nature of science: there are no shortcuts. The sheer hard work and dedication behind seemingly simple scientific breakthroughs are far beyond what most people can imagine. They often oversimplify the achievements of scientists by attributing everything to mere genius.

While this approach presented a slight inconvenience, Aizen saw no better alternative. Unfortunately, acquiring the Shadow Clone Technique wasn't easy. As Kakashi Hatake from the original work pointed out, it was a secret technique once stolen by spies from other villages.

Without access to Shadow Clone Jutsu, Aizen had to postpone his Senjutsu training for now.

Fortunately, his experimentation during this period yielded positive results. Three of the four Shinigami combat skills – Hakuda (hand-to-hand combat), Shunpo (high-speed movement), and Hohou (Movement arts) could be powered by chakra, with the exception of Kido (demonic arts) which seemed incompatible with the lower quality chakra energy. Even some secret Shinigami spells requiring Reiryoku displayed functionality.

Among these, techniques with minimal energy requirements but significant practical value proved particularly useful for Aizen.

In the absence of better options, Aizen could only resort to diligently practicing regular chakra cultivation.

The first step involved gathering resources. Interestingly, the original owner, overwhelmed by grief and hallucinations after his parents' deaths, had never sorted through their belongings.

While internally criticizing the original owner for his foolishness, Aizen wasted no time. After ransacking the house for an extended period, he unearthed a treasure trove!

He discovered a seemingly advanced chakra refinement technique that, upon practicing, accelerated his progress by over twenty times compared to the academy's teachings! Additionally, there was a taijutsu training scroll, which Aizen, armed with the memories of Shinigami Hakuda, knew was no ordinary technique.

But the most surprising find was a collection of various jutsu! This included Water Style: Water Severing Wave, Water Style: Water Dragon Bullet, Fire Style: Great Fireball Technique, Earth Style: Earth Flow Wall, and Lightning Style: Lightning Dragon Bullet – an impressive assortment of B-rank and C-rank ninjutsu techniques.

Even Aizen was surprised to discover several A-level ninjutsu techniques among the stash, such as Fire Style: Blaze Release and Water Style: Hard-Water Blade and various escape techniques, It was outrageous!

He estimated that some average ninja families, even second or third-rate ones in Konohagakure, wouldn't possess such wealth.

"The original owner's mother," Aizen secretly mused, "her background seems far from ordinary."

As for the original owner's father? Well, let's just say Aizen conveniently disregarded him.

The surprises didn't end there. After a period of dedicated training, Aizen was astonished to find that the original owner, whom he'd initially considered a mediocre talent, had managed to increase his chakra reserves to near-Genin levels in just ten days! After that, his physical limitations caused the growth rate to slow down.

Aizen's progress wasn't limited to chakra reserves. He mastered chakra control exercises like tree climbing and water walking with almost effortless ease. Even his escape jutsu practice, particularly his mastery of Water Style techniques, felt surprisingly intuitive.

This rapid growth meant Aizen's strength was practically increasing by the day.