Kenshi closed the door, still confused by Kanao's strange behavior as he sat on the edge of his bed. Deciding to make productive use of his time before training, he accessed his system's skill tree.

"System, didn't I have some kind of 'Check-In' function before? I feel like I'm forgetting something important." 

[Host, as previously stated, the system was not fully integrated into the host initially. Those basic functions and items were provided as rudimentary 'beginner gifts' to assist in dangerous situations. However, now that the system has fully awakened with enhanced capabilities, superfluous features like that have been retired.]

"I see. Well, it's not like that skill proved particularly useful anyway," Kenshi said out loud with a shrug, understanding that the system seemed to operate in mysterious ways, constantly evolving its utilities.

He stood and headed for the mansion's training grounds, his mind buzzing with determination to master the transparent world skill as soon as possible before the duel with Shinobu.

On his way, Kenshi passed by one of the three little girls, Sumi, who had accompanied Tanjiro. Sumi informed him that despite Nezuko's disappearance, Tanjiro had managed to convince Inosuke and Zenitsu to begin training under Kanao's guidance starting yesterday.

Kenshi nodded slightly in acknowledgment, inwardly impressed. He continued on his way, his stride purposeful.

When he arrived at the open training area, he found Mitsuri already waiting, idly practicing a few sword forms alone. As Kenshi approached, she turned and greeted him with a warm smile.

"You're here early! I was hoping we could get some more training in before the match," She said cheerfully.

Kenshi smiled back, resting his hand on the hilt of his sword. "Of course. In fact, I believe you should start using your full strength against me from now on."

He drew his blade slowly, assuming a ready stance. "My abilities have...improved somewhat since our training session yesterday. I'd appreciate you not holding back."

Mitsuri just smiled at the confident proclamation, but then gave a slight laugh and nodded in acceptance of the challenge. Unsheathing her own sword, she wasted no time launching herself forward in a blinding flurry of strikes.

Kenshi's entire world seemed to shift into crystalline clarity as time itself slowed to a crawl around him. Compared to yesterday, he could perceive every subtle muscle twitch that happened in her movements just milliseconds before they had actually occurred.

Sidestepping her opening thrust with total calm, he deftly wove between each subsequent slash of her blade, his form blurring with speed and grace. A hailstorm of sword blows rained down, yet Kenshi twisted and flowed like a tendril of silk caught in the breeze.

A shocked expression appeared across Mitsuri's face as she failed to land even a single blow, her sword cutting through empty space where Kenshi's body should have been again and again. Finally, she disengaged, as she studied him with a newfound glow in her eyes.

"How...?" She murmured, scarcely able to process the spike in his abilities displayed in those few exchanges. "It's like you can anticipate my every move before I make it. Just yesterday your form was still rough, but now your evasion is flawless..."

A slow smile tugged at Kenshi's lips as he straightened from his stance, flourishing his sword. "Indeed, I've managed to accelerate my training in certain areas between our sessions. Though I still have much more to learn in other aspects."

He gestured for her to attack again. "Shall we continue? I want to push these skills as far as I can before the day is done."

Mitsuri's professionalism overcame her surprise as she refocused, launching a fresh barrage of strikes. Again and again their blades danced in a whirlwind of steel, Mitsuri's brow furrowing in concentration as she struggled to land a single blow.

She knew she was not using her breathing technique, neither was he, but it was still a huge improvement from yesterday.

Every time she thought she had him cornered, Kenshi would subtly evade her strikes with eerily unnatural perception. Slowly but surely, however, she began adjusting her techniques, tempering her attacks to become more unpredictable, employing feints and sudden changes of angles.

As the hours wore on, a look of intense focus took root on Kenshi's features. Though evading her assaults had initially been trivial, he soon found himself truly pushing the limits of his abilities to keep up.

'She really is a Hashira for good reason,' he thought, sweat beading on his brow. 'Simply being able to perceive her attacks isn't enough - I have to start anticipating multiple steps ahead while simultaneously watching for openings to counterattack.'

Time and again Mitsuri left fleeting openings, chinks of space that briefly emerged before vanishing just as swiftly. Kenshi attempted to take advantage of them, only to find himself forced back on the defensive as her onslaught intensified whenever he tried.

By the time the sun reached its zenith, both fighters were panting and drenched in perspiration. After some time, they disengaged and sheathed their swords, Mitsuri shooting Kenshi an appraising look.

"Your foresight is remarkable," she admitted, "though you still tend to focus too narrowly on evading rather than creating exploitable openings."

Kenshi nodded. "You're absolutely right. My defense has improved drastically, but I clearly need to work on reading multiple stages ahead while simultaneously seeking out vulnerabilities to counterattack."

He smiled faintly. "Which is precisely why I'm grateful for your assistance in pushing me so hard. I may be able to see the attacks coming, but you've forced me to evolve how I process and respond to that stream of visual data."

"Well, I aim to make you better ," Mitsuri replied with a teasing grin. Her expression turned more serious a moment later. "Though in all honesty, I'm still not certain I'll be able to properly prepare you for your battle with Lady Shinobu after seeing how far your skills have come. She is the Insect Hashira, after all."

Kenshi held her gaze steadily. "Perhaps, but I still need to fight." His eyes burned with determination. "But rest assured, your training has been invaluable so far. I'll continue trying to expand my abilities as much as possible on my own from here."

He gave her a respectful bow. "Thank you again for all your time and guidance these past few days. I'm grateful for it."

Mitsuri studied him a moment longer before smiling and returning the bow. "If you're certain, then I wish you the best of luck, Kenshi. I'll be rooting for you!"

With that, she turned and took her leave, disappearing back in the direction of the mansion. Kenshi watched her go before letting out a slow exhale, stretching his arms over his head.

The rest of the daylight hours were spent refining his techniques through meditation and practice, his mind fully immersed in the goal of beating Shinobu. When night fell, he went back to his quarters.

Settling into a lotus position on his mat, Kenshi closed his eyes and felt the familiar pull of consciousness separating from his physical form. When his lids opened once more, the grey void of the Mindspace surrounded him, vast and empty.

[Host has requested to continue technique training. Initiating Mindscape simulation mode.]

The emptiness around him shimmered and morphed into a rocky valley under a starry night sky. Kenshi flexed his fingers, the phantom weight of his sword materializing in his grip with but a thought.

Though exhaustion should have seeped into his bones, he felt utterly refreshed in his mental projection.

A shadowy figure appeared before him - an opponent crafted from his subconscious knowledge of Mitsuri's and Shinobu's fighting styles. The system told him that from the data he had in his brain, it could make an opponent with similar stats as them, though it might not be perfect.

Kenshi immediately slipped into a defensive stance, sword held at the ready.

The shadow wasted no time, launching itself forward in an explosive blur. Kenshi reacted with absolute perception, his body weaving between each strike with the same graceful ease he had achieved against Mitsuri herself.

But this time was different. His evasions were not merely reflexive reactions; they were conscious choices made after parsing each possible attack and selecting the optimal path of avoidance.

When the imaginary Mitsuri attempted an overhead vertical slash to open up his defenses, Kenshi caught the barest shift of muscle in her bicep and knew the strike was a feint. He pivoted fractionally, allowing the blade to whistle past his face while simultaneously stepping inside her reach.

The shadow faltered, surprised by his fluidity and adaptation. Kenshi pressed the advantage with a lightning-swift thrust, only for his foe to compensate and block at the last possible second with an instinctive guard.

Thus began a furious exchange of thrusts, slashes, and parries, Kenshi controlling the flow of their duel through sheer foresight and prediction.  

Every probing strike served not just to test his opponent's defenses, but to create fleeting openings to gather even more data, expanding his windows of perception.

As the battle raged, blades clashing with thunderous concussive force that sent shockwaves rippling through the valley. To any outside viewer, they would have appeared as blurs of prismatic light, and something too hard to follow with mortal eyes.

Just when Kenshi felt certain victory was within his grasp, the simulation shifted without warning. More shadows suddenly appeared out of nowhere into existence all around him.

Springing back to create space, Kenshi took a defensive stance as he found himself encircled, the shadow of Mitsuri fading into the darkness.

"You want me to fight multiple opponents at once now?" He murmured to the empty airs around him, gauging his gaze against the shadows. "Very well..."

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

The Next Day Evening.

When the appointed time for the duel against Shinobu finally arrived, in the butterfly mansion training area, two people were standing against each other while holding their swords.

Of course, it was Shinobu and Kenshi. There were also quite a lot of audience members: the three little girls accompanied by Kanao and Aoi, the main trio of Tanjiro, Zenitsu, and Inosuke, and Mitsuri had also made time to come and look at the battle.