At first, Kakashi was somewhat skeptical about Naruto's claim of crossing worlds. But when he personally witnessed Naruto instantly 'disappear' in the middle of the night, he had no choice but to believe it.
This wasn't the dispersion of a Shadow Clone Technique or something like the Body Flicker Technique… it was a complete and utter vanishing act, with no Chakra fluctuations whatsoever.
What's more, Naruto had been sound asleep at the time.
After Naruto disappeared, Kakashi comforted the others while personally conducting an exhaustive search of the surrounding area.
The results were pretty much what he had expected.
With the physical capabilities of a Jonin, Kakashi could cover a radius of twenty or thirty miles in a single day.
But no matter how far he ran or even after deploying his Ninken, he couldn't find a single trace of Naruto's movements.
That was when he recalled the 'truth' Naruto had initially confided in him.
At this point, he has to believe it even if he doesn't want to.
When Naruto returned, Kakashi immediately greeted him, even helping him come up with a plausible excuse.
After all, the reason Kakashi had given Team 7's two younger members to placate them was, "That kid got drunk and ran off on his own, I couldn't even stop him", and "Don't worry about him, do you really doubt his abilities?"
As a result, neither Sakura nor Sasuke had paid it much mind.
The only one truly anxious was Kakashi, who knew the truth.
Because he had no idea when, or if, Naruto would return.
What if he didn't come back? On his very first mission outside as their Jonin Instructor, he'd managed to lose the Nine-Tails' Jinchuriki, the Fourth Hokage's son. No matter how convincingly he explained it, there was no way Hiruzen would believe him.
"Hahaha, talk about embarrassing… I got so drunk I fell into the river and got swept pretty far downstream. I just barely made it back."
Naruto also scratched his head and told a little lie very naturally.
The two of them acted so naturally that Sakura and Sasuke didn't suspect a thing, simply greeting Naruto before the group headed back to the village to pack up.
The mission in the Land of Waves was complete, and today was the day they'd return.
"Goodbye!"
"Goodbye! Come visit again sometime!"
"Goodbye, Naruto-nii-chan!"
A crowd of people waved enthusiastically at Team 7, their faces filled with gratitude.
Just the day before, while searching the area for Naruto, Kakashi had also stumbled upon the abandoned base of Gato's organization.
There, he'd found the man's corpse.
Soon after, news of Gato's death spread throughout the village. Though the villagers didn't know who had killed him, they had their suspicions.
Because of this, their gratitude toward Naruto and the others grew even deeper.
After saying their goodbyes to the people of the Land of Waves, the journey back was peaceful, with no foolish assassins daring to cross their path.
This gave Naruto the perfect opportunity to continue training Sasuke.
Before his trip to the One Piece World, Naruto had already sensed that Sasuke's physical conditioning might be sufficient to awaken Haki.
Now was the time to put that theory to the test.
As for Armament Haki, that could only be developed through long-term physical training, so there was no need to rush it.
First, he had Sasuke experience the effects of Observation Haki.
To give him a feel for it, Naruto had Sasuke attack him while he only dodged, never counterattacking. He even restricted himself to a small area, limiting his movements to minor shifts within that space.
The sheer number of restrictions made Sasuke feel like Naruto was underestimating him, and his previously subdued pride began to resurface.
"Hmph, don't blame me if you get hurt."
Sasuke muttered, trying to salvage some dignity.
But what followed completely shattered his confidence.
No matter whether he used Taijutsu or Ninjutsu, Naruto effortlessly dodged every single attack with minimal movement… even strikes from behind.
"Damn it…"
Sasuke was drenched in sweat, panting heavily from exhaustion, while Naruto remained perfectly calm, standing still with his eyes closed.
Watching from the sidelines, Kakashi couldn't help but be amazed.
This kind of omnidirectional, blind-spot-free attack prediction was, in some ways, comparable to the Byakugan possessed by the Hyuga Clan.
But the Byakugan was a Kekkei Genkai unique to the Hyuga Clan, unless someone stole and transplanted their eyes, there was no way to replicate it.
Because of this, the Hyuga Clan had even developed the so-called [Caged Bird Seal], ostensibly to protect their bloodline. It was a tragic state of affairs.
And now, Naruto was claiming that a similar ability could be acquired through training.
If the Hyuga Clan ever found out, Kakashi wondered how they'd react.
Setting aside Kakashi's thoughts, after exhausting every possible method and still failing to land a single hit, Sasuke finally grasped just how powerful this ability called Observation Haki truly was.
So, in Sasuke's daily training routine, aside from his usual Ninjutsu practice and physical conditioning, he now had an additional task: Haki training.
The training method is also extremely simple and crude.
Naruto would stand behind Sasuke and attack him with a wooden stick.
Sasuke had to rely purely on his instincts to dodge.
Once he mastered that, the next step was blindfolding him while Naruto's attacks became even more silent and unpredictable.
Soon enough, Sasuke, who had been feeling a bit smug after quickly completing the first stage, was swiftly humbled.
Before, he could at least rely on faint sounds of wind or changes in light to assist him. Now, he had nothing.
There was complete darkness before his eyes and silence all around.
Only the sharp, precise strikes of the wooden stick against various parts of his body.
Before long, he was covered in bruises, which made Sakura, who was standing beside him, feel distressed.
"Naruto..."
She wanted to intervene, but Sasuke himself refused her concern.
Fortunately, Naruto knew how to hold back. Despite being struck hundreds of times, Sasuke never suffered any serious injuries… just bruises.
But the pain is very real.
And yet, this only proved just how much of a genius Sasuke truly was.
After just a short period of this grueling training, he showed remarkable improvement.
It could be because the Uchiha Clan was born with the Sharingan and thus had strong mental prowess, or maybe it was something else entirely.
Naruto had originally assumed it would take Sasuke at least a month to even grasp the concept of Observation Haki. And yet, after just a few days of focused training, the kid successfully predicted one of his attacks.
Even though he failed many more times afterward-
There is no doubt that state of 'knowing without seeing' was the budding form of Observation Haki.
Naruto believed that, at this rate, it wouldn't be long before Sasuke could master Haki on his own, even without his guidance
And that… would definitely give his 'bastard of an older brother' a little surprise.
At least, that's what Naruto thought.
Of course, this was also because he didn't fully understand just how strong Itachi really was.
If he knew the sheer gap in power between Itachi and Sasuke, he might've doubled, no, tripled the intensity of the training.
Then again, this also indirectly confirmed one of Naruto's theories.
…That is, Haki was derived from the body's spiritual and physical energies, meaning it could be compatible across different worlds.
However, this also raised a new question.
Because according to the textbooks, Chakra is also a unique energy formed by extracting the body's life force and combining it with focused mental training.
So fundamentally speaking, the two should have been very similar, possibly even interchangeable or complementary.
He himself was living proof of that.
'And yet, in the other world, even someone as strong as Captain Roger couldn't produce even a trace of Chakra.'
'Did that mean the birth of Chakra depended on some other factor?'
With this in mind, Naruto decided he'd conduct some experiments later when he had the time.