Kenshi opened his eyes to find Shinobu looking at him with wide open eyes, her jaw practically unhinged in disbelief. "Why are you looking at me like that?" he asked, raising an eyebrow.

The violet-haired beauty seemed to struggle for words, her eyes flickering over his changed appearance. She lifted a trembling hand and pointed at his face. "K-Kenshi... Your..."

Frowning, he reached up to touch his own face. He immediately sensed something different – the futures felt sharper, more defined. His fingertips traced over higher cheekbones and a more angular jawline.

Feeling baffled by this unexpected development, he swiftly sat up straighter in the bed. That's when he truly realized just how much had changed.

With a slight widening of his eyes, Kenshi glanced down at his hands. His fingers seemed longer, the muscles in his arms more toned and pronounced than before. He slowly rose to his feet, and waves of confusion washed over him as he noticed his increased height, now several inches taller.

He caught a glimpse of his reflection in the glass window and did a double-take. Staring back was a striking young man with delicate, almost ethereal features and brilliant purple-colored hair spilling past his shoulders in silken waves.

He didn't look all that different, except for the hair color change. But Shinobu, who spent so much time with him, could spot all the small facial differences.

'The system... when it mentioned reconfiguring my body, was this what it meant?' Kenshi muttered under his breath, studying his transformed self with a mixture of awe and confusion.

"K-Kenshi... what happened to you?" Shinobu finally found her voice, brows furrowed with deep concern.

He was speechless. He didn't know how to make sense of what just happened, let alone explain it to her. She had witnessed something unbelievable. He hoped she wouldn't freak out and call him a demon now.

Before he could formulate a response, Shinobu spoke again, her analytical mind racing. "I suppose it must be a side effect from that strange spider venom. Demon poisons can manifest in unpredictable ways... They can also affect one's hair color and so on." She chewed her lip pensively before straightening with resolve.

"Regardless, we should examine this more thoroughly. Here, sit down and let me draw some blood samples for testing." The Insect Hashira guided Kenshi back onto the bed and quickly retrieved a case of medical supplies.

As he watched her hands prepare syringes and vials with a look of intense concentration, he couldn't help feeling a surge of affection for his mentor. She was too good to him. A smile tugged at the corner of his lips.

Kenshi remained silent as Shinobu deftly drew several vials of his blood. Once finished, she gathered the samples with a low murmur. "I'll analyze this right away. Try not to strain yourself in the meantime."

With a reassuring pat on his shoulder, she swept from the room like a butterfly.

Left alone with his thoughts, Kenshi exhaled a deep sigh and ran his fingers through his newly long hair. He had no idea what curveballs this world still had in store for him.

Just then, the muffled sound of sobbing reached his ears. His brow furrowed as he stood and followed the noise to the next room over.

He slid open the door to find a yellow-haired boy curled up in a corner, shaking and crying as tears streamed down his face.

On the opposite side laid Inosuke, who was lost in exhausted sleep, oblivious to the person crying in the room. Kenshi looked around and found no sign of Tanjiro.

Looking at Zenitsu and his crying, it was hard for Kenshi to feel sympathy given that Zenitsu's character was mostly shown in a comedic light.

At the sound of the door sliding, Zenitsu startled and looked up with red, puffy eyes.

"You're the one that rescued me, big brother! Nezuko is GONE!" shouting that, he suddenly launched himself forward with surprising force, snot and tears flying as he aimed straight for Kenshi's chest.

Moving back a little, Kenshi planted a firm kick squarely in Zenitsu's stomach, knocking the wind out of him and sending him sailing backwards. He landed on his bed, wailing incoherently about losing his precious Nezuko-chan.

Kenshi stepped back, having no patience for Zenitsu's usual antics – especially not under such grave circumstances. Wordlessly, he rose and made his way out of the room, ignoring the boy's pitiful sobs echoing behind him.

His steps led him toward the outdoor training area, guided by the sharp ringing sounds of a blade cutting through the air.

His steps suddenly stopped as he saw the red-haired boy swinging his blade in the warm sunlight.

It was Tanjiro.

The young demon slayer stood alone, practicing slashing and cutting with his sword. Every so often, a stray tear would escape the corners of his tightly shut eyes, only to be swiftly wiped away before resuming the training.

Watching him for a few moments, Kenshi found himself deeply impressed by Tanjiro's stoic perseverance, even after what happened to his sister, he was still training.

"Well, that's one of the perks of being the protagonist!"

Rather than interrupt Tanjiro's practice session, Kenshi decided to go to a quieter space to check out his new abilities.

He settled beneath the canopy of a willow tree overlooking a small koi pond at the edge of the Butterfly mansion's beautiful landscaped gardens.

He closed his eyes only to opened them in pitch black darkness.

[System detected that host entered the Mindspace] The robotic voice resonated around him amidst the void.

[Does host want me to proceed with your first training exercise?]

"First training exercise? Is there a training plan?" Kenshi asked.

[System's current top priority is to make host a Hashira-level swordsman, so the system indeed prepared a training schedule for the host, and these training exercises are also given in the form of missions.]

[Mission: Blade Fundamentals]

[Requirements: Successfully complete 10,000 sword swings with perfect stances.]

[Reward: Unlock the Demon Slayer Mark.]

Kenshi's eyes narrowed intently as he processed the information. Ten thousand swings sounded daunting, even if it wasn't in the real world; it would still take a lot of time. And the reward for the mission caught his attention.

"What? A Demon Slayer Mark as a reward? Wouldn't it just kill me in my twenties? What are the benefits of having it?" He asked.

[The Demon Slayer Mark was a rare and risky phenomenon that occurred when a Demon Slayer pushed their limits beyond the normal boundaries of life. It involved channeling one's entire being – body, mind, and soul – into a single point of attack that transcended the physical realm. It was a gamble of sacrificing one's existence for transcending human boundaries.]

[However, this system's variant has a unique feature that simplifies and improves the way you activate the mark. Since the host did not acquire it by himself and the system was giving it in the form of a skill, the host doesn't need to worry about it taking away his life span as the host can enable and disable the skill through the system.]

"Hmm." Kenshi thought for a moment before nodding.

"Another question, what do you mean by a perfect stance?"

[It will be a good thing if the host performs each motion with perfect form. Speed and strength alone are meaningless without a solid foundation of fundamentals.]

"Understood."

'Damn, I'm feeling like the system is being more of a teacher than my actual teacher.' He thought to himself as he continued.

[The system would like to inform the host that there will be a time dilation between the Mindspace and the world. Particularly, time flows 1 time slower inside the Mindspace. For example, if 2 hours pass outside, 1 hour will pass in the Mindspace.]

[If the host wants to improve the time dilation period, the system suggests that the host find out more about the Mindspace and how to control it.]

[This capability represents a significant advantage for rapidly training in techniques. Use it wisely, host.]

A slow grin spread across Kenshi's newly striking features as he summoned his sword, the Thousand Blade Senjin.

He found that he could create small items out of thin air, but if he wanted to create something big, he needed more control over the Mindspace.

Feeling the tangible weight of the hilt in his grasp sparked a surge of excited determination within him.

"Then let's not waste any more time."

As soon as the words left his lips, Kenshi launched into a blistering series of sword katas and forms ingrained from countless hours of physical repetition in the real world. Each strike sliced through the airy Mindspace void with razor precision and intense focus and muscle memory.

Slice after slice, parry after parry – his blade created a dazzling pattern of lethal curves and swift counters. Kenshi felt his forehead dripping with imaginary sweat as he tested the boundaries of his mental power, every move demanding precise control and timing.

He couldn't afford to mess up – even the tiniest mistake or deviation from the textbook form would ruin count. Kenshi was determined to keep his focus and form sharp, blocking out everything else with each slash of the sword.

Five hundred... One thousand... Two thousand...

He was on his three thousandth strike, and he felt like his ghostly arms were about to fall off. His muscles were on fire, and his lungs were screaming for air. He had to shout every time he swung his sword, but his voice was hoarse and weak.

He didn't stop or slow down, though. He didn't care about the pain or the fatigue. He only cared about perfecting his form and his skill. He wanted to be the best swordsman ever, even if he was already dead.

Seven thousand... Eight thousand...

His entire existence became an extension of the blade, willpower and sword fused into a single unstoppable force.

And finally!

Ten thousand!

With a resounding final kiai shout, Kenshi delivered a devastating double-handed overhead strike straight through his imagined foe. The thunderous hum of his blade shuddered to a halt as its momentum dissipated into the darkness.

Kenshi was stuck in that pose for what felt like forever – his chest rising and falling fast, his forehead dripping with sweat, his sword pointing out in a smooth final move that Shinobu had taught him. He could sense his mind about to break into tiny pieces from the strain.

[Mission complete. Technique assimilated.]

The system's crisp voice cut through the ringing silence. Kenshi felt a surge of relief and pride at those words, finally permitting himself to straighten and sheathe his blade.

[The Demon Slayer Mark has been successfully unlocked and integrated into your skill tree, host.]

Kenshi saw the shimmering screen with a skill tree appear in front of his eyes. He had just unlocked a new skill, a rare and powerful one, that glowed brightly at the top of the tree.

He felt a surge of excitement and curiosity as he explored the holographic wireframe and new abilities, wondering what new possibilities awaited him.

[You now possess the ability to temporarily assume your body's utmost offensive and defensive capabilities by invoking the mark.]

[However, access to the skill remains limited – the mark can only be sustained for 5 minutes when activated, and physical damage would occur if overextended. Total duration increases with experience and mastery.]

Kenshi absorbed the accompanying information torrent, immediately comprehending the finer nuances involved in utilizing such an advanced skill.

"This is incredible..." He murmured, watching the screen.

"Maybe I should try this skill now!" Kenshi suddenly realized that he had spent more than 3 hours in this space, which meant that in the outside world, 1 and a half hours had passed, and all this time he was probably laying under a tree.

Putting aside the testing, he immediately closed his eyes so he could get out of the Mindspace.

------------------

In the outside world, Kenshi suddenly opened his eyes, and two pink ponytails were lolling in the air right in front of his eyes.

"Hi, I was waiting for you to wake up," Mitsuri said with a smile, standing beside him and leaning down towards him.