Nanobots

On the training ground, Kakashi's entire body turned into a streak of lightning as he darted back and forth, continuously charging at Nico Robin.

The Lightning Release Body Refinement Technique not only improved Kakashi's physical abilities, but also gradually increased his resistance to and control over Lightning Release. As those two abilities improved, Kakashi could endure more Chakra through ordinary cellular stimulation and burst forth with even greater speed.

If Kakashi, transformed into a streak of lightning, was a high-speed, high-agility assassin, then Nico Robin standing opposite him was a mage who had learned multi-casting!

Dozens of arms sprouted from behind Nico Robin, each hand forming different Hand Seals, giving her something of the appearance of a Thousand-Armed Guanyin.

Yet as so many hands formed Hand Seals at high speed, Nico Robin's Ice Release ninjutsu also became unimaginably fast. She could instantly cast three or five ninjutsu at once, enough to catch any ninja opponent completely off guard.

Ice Release: Ice Rock Dome No Jutsu, Ice Release: Swallow Blizzard, Ice Release: Dragon Shattering Fierce Tiger...

One Ice Release ninjutsu after another forced Kakashi to keep evading Robin's attacks, while the summoned walls of ice rendered Kakashi's attacks almost entirely ineffective.

Although Kakashi's current strength allowed him to shatter a wall of ice in less than a second, Robin could summon two or three more identical ice walls in that same amount of time.

"Miss Nico Robin, isn't your Chakra amount a little absurd? Even the Kage of the Five Great Shinobi Villages couldn't squander Chakra like you do."

Kakashi retreated from Robin's attack range at tremendous speed. A troubled expression appeared on his face as he panted and complained.

Since the battle began, his opponent had thrown out at least dozens of B-Rank and C-Rank Ice Release ninjutsu. By all reason, even a Jonin-level ninja should have nearly exhausted their Chakra after using so many techniques.

Yet Nico Robin possessed at least over a dozen times as much Chakra as an ordinary Jonin. And as someone from the Pirate World, not only was her stamina far beyond that of ninjas in this world, but her recovery ability was also exceptionally powerful.

Even ordinary people in the Pirate World could recover from near-fatal injuries within days as long as they were bandaged, fed, and cared for properly. Reflected in Chakra, that meant Nico Robin's Chakra recovery speed was more than a dozen times faster than an ordinary ninja's.

Robin could fully recover the Chakra spent on attacks of that level in less than five minutes, so the pressure on her was nowhere near as great as Kakashi imagined.

"Really? Maybe your Chakra is just too meager, Kakashi. You should train more. It would be such a pity if you became weak while you're still so young."

The black-bellied Nico Robin replied to Kakashi with a smile.

Kakashi's expression darkened slightly. At only thirteen or fourteen, he was not yet the thick-skinned, washed-up ninja who, more than a decade later, could openly use little yellow books to test his students. Being told he was weak still bothered him a little.

"Alright, it seems this test was quite successful. Kakashi, you've grown much stronger too!"

Vegapunk, who had been standing nearby and staring at his instruments while analyzing the pair's combat data, shut the instrument in his hand and smiled.

"Judging from that battle, Robin, you've now become familiar with your new fighting style. If you combine it with the restraining abilities of the Flower-Flower Fruit, then there are probably very few people in this world who could be your opponent."

"As for Kakashi, your speed has increased by approximately 13%, while your Chakra amount has risen by 50%. Although that is partly because progress is fastest at the beginning of training, by this estimate, the Lightning Release Body Refinement Technique can increase your Chakra by at least twofold and raise your speed by roughly 25% to 30%."

That effect was already astonishing. Although the amount of Chakra a ninja possessed was not the sole factor determining combat strength, it was still an important one. Especially for someone like Kakashi, who was already short on Chakra, doubling his reserves could greatly enhance his strength.

For that reason, Kakashi's expression improved considerably, and he no longer pursued the matter of Vegapunk treating him as an experimental subject—most importantly, there was no point in pursuing it. None of the three before him was simple. If they got serious, even Nico Robin, the weakest among them, could easily suppress Kakashi.

Konohagakure held little deterrent power over the three of them, who were now in faraway Snow Country. The fact that Konohagakure had not sent anyone to investigate despite Kakashi being gone for so long also showed that even the strongest shinobi village had places beyond its reach.

After all, the distance between them was so great, with Water Country in between. By the time Konohagakure fully reacted, one or two months would likely have passed, and taking concrete action would probably take even longer.

That was only because Kakashi was relatively important. If it had been some ordinary Chunin instead, Konohagakure probably would not have sent anyone to investigate Snow Country, such a remote backwater, at all.

However, Vegapunk estimated that it would not be long before Konoha came knocking. In other words, Kakashi, this useful tool, could no longer be used for much longer.

After completing the combat analysis, Vegapunk took out another bottle of Nutrient Solution and handed it to Kakashi.

Ever since Kakashi began training in the Lightning Release Body Refinement Technique, Vegapunk had continuously supplied him with Nutrient Solution. Kakashi did not particularly suspect anything. The Ninja World also had things like Secret Medicine, and high-energy nutritional supplements with special effects were nothing rare.

As he watched Kakashi drain the Nutrient Solution in one gulp, Vegapunk sensed that the Nanoparticles controlled by the Quantum Brain within it had spread throughout Kakashi's entire body. A faint smile appeared at the corner of his mouth.

In the Ninja World, the various methods Vegapunk had originally developed in the Pirate World for controlling people had become largely ineffective.

For example, bombs implanted in the brain could not be concealed from the Byakugan at all. Brainwashing through the Quantum Brain was also difficult to hide from ninjas with Chakra. Cloning and replication were still quite effective on ordinary people, but for ninjas with Chakra, clones were far inferior to the Main Body.

This extraordinary power known as Chakra was not only connected to the body, but also had a certain connection to the spirit. Even for clones, the nature of their Chakra differed from that of the Main Body.

For that reason, over this period of time, Vegapunk had relied on the power of the Quantum Brain, his own Observation Haki, the instruments in the Laboratory, and his ability to control Magnetic Fields and electric currents to finally produce a batch of tiny Nanobots.

These Nanobots were minuscule. After entering the human body, even abilities such as the Byakugan could not detect them. Controlled by the Quantum Chip in Vegapunk's brain, they could be used perfectly as a hidden means of controlling others.