[Perception Training]

Entering the sparsely populated trainee dormitory, they arrived at a modest wooden door marked 'Room 19-A.'

"Catch this, it's your room key," Rinkusu said, tossing a silver key into Sasuke's hand.

Sasuke effortlessly caught the key, swiftly unlocking the door before him. As he swung it open, revealing the contents of the room.

The room was about 10m² in size and akin to the size of his previous student accommodation when he was Ren, the walls were adorned with plain, yellow wallpaper.

It featured a single bed snug in one corner, opposite the bed sat a wooden desk and a small bathroom was also located within a smaller room off to the side of his room.

"How does it look?" Rinkusu inquired.

"It should do," Sasuke answered him.

"Well, that's what matters, isn't it? Anyway, I'll guide you through the other sections of HQ, and then you'll kick off your first day of training," Rinkusu added.

Sasuke nodded in acknowledgement, proceeding to unpack his Anbu equipment and arrange it within the room. After ensuring the door was securely locked, he and Rinkusu swiftly exited. Rinkusu then led Sasuke through the Mission and Hospital Sections before arriving at the Training Section.

A series of training grounds surrounded a sizable, dome-like grey building at the centre of the section. Dark silhouettes darted around within the training grounds, engaged in sparring sessions or practising Jutsu independently.

"Most Anbu use those training grounds for sparring and honing their skills. As a recruit, your primary training will take place within this building," Rinkusu explained, gesturing to the structure in front of them.

"Here, you'll work closely with dedicated mentors specialising in specific areas. It's crucial to understand that Anbu members, like yourself, are handpicked for their skills. You won't receive direct training in Ninjutsu, Genjutsu, Taijutsu, and the like.

The belief is that you are capable of honing those skills independently or seeking assistance from comrades. However, mission credits can be earned and exchanged for certain techniques, a decision reserved for Lord Hokage." Rinkusu clarified as he began shedding light on why Anbu members weren't trained directly in terms of combat ability.

Sasuke understood that whilst this approach served as a means for individuals like Hiruzen to cultivate heightened loyalty within the Anbu, it also allowed members to acquire Jutsu when their credit reached a sufficient level- allowing for members of Anbu to receive a chance at increasing their strength.

Additionally, Sasuke realised that Hiruzen could grant access to powerful Jutsu to those loyal to him, irrespective of their credit, given his final decision-making authority in such matters.

Nevertheless, Sasuke refrained from delving too deeply into the moral implications of this system, primarily because it seemed to work in his favour.

Given his youth, Hiruzen likely believed that Sasuke would be more susceptible to loyalty through the relatively straightforward means of granting him Jutsu and other similar incentives. 

Rinkusu guided Sasuke into the building, approaching the receptionist. She was a young woman with straight, pink hair and was clad in a dark blue outfit.

"I'm here with a recruit," Rinkusu informed the receptionist.

"Oh? Is it Sasuke Uchiha?" The receptionist replied as she sneaked in a glance at Sasuke before adding, "Okay, he'll start with Perception training today with Usagi. I'll take it from here."

"Kid, remember that if you need any help, you can come find me," Rinkusu assured Sasuke before waving at the boy and exiting the building.

'But he didn't say where to find him?' Sasuke pondered silently before being guided to a room labelled 'Perception Training - Room 2' by the woman.

"Remember to come to this building every day, I'll be guiding you to where each of your classes will be. Today will be Perception training for you, Stealth training tomorrow, and Assassination technique training the day after tomorrow. Then you will begin the cycle again with Perception training. You will train six days a week and have one day off to recover from mental fatigue in the lounge," the woman explained to Sasuke before she opened the door.

"Usagi! You have a new student here," the receptionist called out as she opened the door to the room.

The room was quite spacious, devoid of any furniture, and predominantly empty. The grey wallpaper added to the somewhat sombre atmosphere that enveloped the room.

The floor was scattered with various tools, from shurikens to small steel balls, all featuring Fuinjutsu patterns on them. In the room, a woman adorned in traditional Anbu attire with curly brown hair and a white rabbit mask, marked with several blue patterns, caught Sasuke's attention.

She was crouched on the floor, seemingly organising the equipment that littered the ground.

Upon hearing the receptionist's voice, she stood up and glanced at her and then shifted her gaze to Sasuke before replying, "Got it."

The receptionist awkwardly chuckled before saying, "Hah, Usagi, you're still so uptight. Anyway, Sasuke, I'll leave you here in her care. Don't try to annoy her, though. She is very scar-" Before she could finish her words, a pointed glare was directed at her.

*Cough*

"I'd better get going," she muttered quietly before dashing out of the room, closing the door as she left.

An uncomfortable silence soon enveloped the room, marked by two pairs of scrutinizing eyes locked onto each other. Breaking the silence, the woman spoke, her words cutting through the quiet air, "I'll be your perception teacher. You can call me Usagi-sensei."

Internally, Sasuke winced at the idea of addressing someone with that name, especially considering the evident coldness emanating from the woman standing before him.

"Hello, Sensei. I'm Sasuke Uchiha, but my code name is Okami." Sasuke replied as he slyly avoided calling her Usagi-sensei.

"Good, I'll call you Okami from now on. Before I delve into training you in Perception, I will briefly describe what Perception even entails and why you are going to learn it," she explained in her characteristically cold voice before continuing.

"Perception is the art of sensing the subtle currents of chakra in one's surroundings. As you progress, you'll master the skills to pinpoint and track specific chakra signatures as well as gauge the overall chakra capacity of an individual. This ability is fundamental for Anbu operatives, facilitating intel gathering and threat identification.

Keep in mind, Okami, that just like your other senses, chakra perception can and will be deceived by enemies in your future missions.

Typically, individuals sense chakra by releasing a pulse of their chakra into their surroundings.

This pulse will interact with the chakra in living beings around you. Picture it like how bats sense their surroundings through echolocation, but we use chakra instead of sound." Usagi concluded her explanation and then inquired, "Any questions, Okami?"

"Are there other methods to sense an individual's surroundings?" Sasuke pondered for a second, before asking.

"Interesting question. Yes, there are many other methods to sense one's surroundings. Off the top of my head, the most common method other than the basic method is something unique to powerful ninjas or those who train in spiritual arts extensively.

People like this will possess a strange instinct towards danger and people in their surroundings. Of course, they can't actively use this and it isn't as versatile as the standard perception method as they can't figure out chakra signatures or chakra capacity of individuals," Usagi explained

"So that's what happened to me..." Sasuke finally understood why he was able to sense the surveillance placed on him all those years ago.

"Thank you, sensei," Sasuke sincerely expressed his gratitude to Usagi as she imparted her knowledge to him.

"Well, we can now begin your training. See these little balls on the ground here? They have a tiny amount of chakra sealed within them. To start this off, I will try to help you sense them."

Usagi instructed Sasuke to sit on the floor and stood behind him, guiding him on the precise method to release a pulse of chakra as she placed several of the balls 1m away from Sasuke.

Sasuke took a deep breath, closed his eyes, and attempted to clear his mind. He then released a chakra pulse, sensing an image gradually forming in his mind.

The room around Sasuke transformed into a dark, empty void, yet he noticed vague, flickering blue lights emerging around him.

'Is that chakra?' Sasuke wondered.

He opened his eyes and found Usagi standing in front of him, her expression tinged with surprise.

"You saw it?" Usagi's voice held a hint of astonishment. As a sensory ninja, she could detect the chakra pulse Sasuke had just unleashed.

"Do you mean those blurry, glowing blue lights? If so yes," Sasuke replied, slightly confused by her reaction.

Hearing his words, Usagi despondently spoke to herself aloud, "Five days, it took me five days to achieve that... Hell, I was considered a genius for that too!"

Most Anbu recruits would have taken at least ten days to release that chakra pulse and sense surrounding chakra signatures. They would then spend the rest of the month refining their skills, by trying to count the number of chakra signatures and remember the target's specific chakra signatures. 

'It's honestly my first time seeing someone so talented; I'll probably have to change the training schedule at this rate,' she sighed gently, as she thought to herself.

"Change of plans. By the end of today, we will aim for you to be able to sense the number of chakra signatures surrounding you," she addressed Sasuke. 

The rest of the day was consumed by the intense training session, Sasuke dedicated himself to absorbing as much knowledge as possible from the lesson.

By the end of the session, he could count the number of chakra signatures in his sensory field, which he managed to extend to around a 5-meter diameter.

After the training session with Usagi, Sasuke returned to his room following dinner at the lounge cafeteria. Before he went to sleep he refined his chakra and tried a new form of training.

Swiftly rolling up his left sleeve, Sasuke channelled his chakra into his right index finger. With care, he slowly began inscribing a series of Fuinjutsu seals onto his skin.

Soon, the world around him erupted into a brilliant white light, and Sasuke found himself suspended in the air. Glancing back, he observed his physical body lying motionless on the bed. Inwardly, he mused, 'I might not ever get used to this.'

It was Sasuke's first attempt at conducting this experiment on himself. Previously, he had applied this seal to animals within the Forest of Death, trying to discover the duration it took for the symbols to fade from their bodies.

Sasuke quickly dispelled his distracting thoughts as he began meditating. However, after ten minutes had passed, he opened his eyes and observed his physical body. Witnessing the Fuinjutsu seals on his body gradually fading, he hovered over himself, eventually merging into his physical form.

"I can only do this for ten minutes..." Sasuke muttered to himself. He realised the need to increase his proficiency with this seal if he ever aimed to extend the duration of his presence outside his body.

Though this method may seem reminiscent of soul-refining exercises from Xianxia novels, Sasuke was aware of the stark difference between them. He didn't rely on absorbing any special energy when doing this; the mere act of existing outside the physical body placed an undue strain on the soul.

This stress placed on the soul would gradually dissipate and heal with sufficient rest.

Repeatedly engaging in this practice in measured intervals would enable the soul to adapt to the pressure and gradually grow over time. Subsequently, one could slowly extend the duration of the soul staying outside the body to increase the strain on the soul.

As Sasuke contemplated this particular exercise, a lingering concern surfaced in his mind.

If someone dedicated enough time and effort to this exercise, could they potentially transcend their physical body and exist as a soul? 

Did someone like that ever exist in the past?

Images and figures began to flash through Sasuke's mind.

'The Sage of Six Paths? No, he is undoubtedly dead. What materialized in the anime was merely a chakra manifestation, born from the vast chakra he wielded during his lifetime.'

'The Tailed Beasts? They are essentially chakra amalgamations given life by Hagoromo; it wouldn't be right to consider them spiritual entities in that regard..'

(Author: Hey, for those who don't remember. Hagoromo = Sage Of Six Paths)

'So, is there any entity in the Naruto world that exists without a physical body, seemingly manifesting purely as a soul?'

Suddenly, Sasuke brows creased as a certain figure appeared in his mind.

'There is. There is one entity that perfectly fits these exact characteristics.'

'The Shinigami...?'