Isekai Adventure in the Dimensional Cube

The appearance of the Red Wave did not surprise Tanjirou.

After all, a Swordsman who had mastered the breathing techniques could conjure fire, water, and wind. It was merely a matter of glowing, nothing to be amazed at.

Tanjirou merely watched nervously and expectantly as the Red Wave flowed like water from Reiger's hand resting on Nezuko's forehead to the girl, gradually covering her entire body.

Reiger controlled the precision of the disintegration magic to avoid any mishaps, such as accidentally disintegrating someone, which would truly be taking the joke too far.

Reiger targeted only the foreign bodies within Nezuko, specifically the demon blood that did not belong to her human body.

In such a situation, Nezuko suddenly began to convulse.

"Hmm..."

A sound resembling pain leaked from Nezuko's mouth, clamped around a bamboo tube.

Tanjirou nearly couldn't restrain himself from rushing forward, but ultimately, he held back and chose to trust Reiger.

However, Nezuko trembled even more violently, the sound from her mouth growing increasingly painful, eventually causing smoke to emit from her entire body.

"Nezuko!"

Tanjirou eventually couldn't hold back any longer and called out.

Unfortunately, at this moment, neither Reiger nor Nezuko had the spare strength to attend to Tanjirou.

It seemed Nezuko was enduring inhuman pain, causing her arms and legs to flail unconsciously.

"Tsst!"

Her sharp nails unintentionally scratched across Reiger's hand, leaving a bloody mark on his arm.

Reiger furrowed his brows slightly but did not stop, calmly continuing to release the waves of disintegration magic.

He had previously thought it too simple, assuming that extracting Kibutsuji Muzan's blood from inside Nezuko would allow her to revert to human form. However, Kibutsuji Muzan's blood had already fused deeply with Nezuko's flesh, completely transforming her physique; it was not so easy to disintegrate.

Reiger needed to identify and disintegrate only the "non-human part" inside Nezuko, without harming her, a task as difficult as finding a splinter in an invisible cell, which seemed nearly impossible.

In the past, Reiger indeed would not have been able to accomplish such a task.

Fortunately, Reiger had now awakened to a transparent understanding of the world, able to directly see through a human body and observe every muscle and every blood vessel's activity. With more concentration, picking out the "splinter" embedded in the flesh and cells was not impossible.

Thus, Reiger completely shut off all his senses, except the bare minimum necessary, and concentrated intensely on meticulously observing every cell and piece of flesh in Nezuko's body.

Then, Reiger identified all the foreign substances inside her and disintegrated them with magic.

This step was tedious, lengthy, and tested one's patience.

At least half an hour passed, and Reiger was still continuing, while Nezuko still struggled painfully, indicating the difficulty of this task.

It was only after half an hour that the smoke emitting from Nezuko's body began to lessen, and both the painful sounds and her struggle gradually decreased.

About an hour later, the Red Wave disappeared, and Nezuko had completely returned to calm.

"Phew..."

Reiger then exhaled, standing up.

"How, how did it go, Mr. Reiger?"

Tanjirou, who had not blinked throughout the entire process, stared intently and rushed to ask as soon as it seemed Reiger was finished.

However, Reiger did not answer; he just gave him a slight smile and turned to leave the place.

"Mr. Re..."

Tanjirou subconsciously tried to speak, but in the next second, he was interrupted by another voice.

"Brother..."

When this faint, mumbled voice echoed in the room and reached Tanjirou's ears, he felt as though he had been struck by lightning, frozen in place.

After a moment, tears once again welled up in Tanjirou's eyes.

......

As Reiger left the place where Tanjirou was staying, upon reaching the door, he immediately heard the loud cries coming from inside.

"He's had it hard too."

Reiger sighed lightly and did not hurry to leave, instead waiting there.

This waiting turned into another half hour.

Then, Tanjirou emerged from his lodging, wiping his tears.

"Thank you, Mr. Reiger, I truly appreciate it!"

Upon seeing Reiger, Tanjirou, without a second thought, ran up to him and repeatedly bowed deeply.

"Enough, you've already thanked me many times."

Reiger helped Tanjirou up, preventing him from bowing further.

"No, no amount of thanks can express how I feel right now." Tanjirou, with a look of immense relief and tear-filled eyes, said, "Nezuko is the only family I have left. I've been really anxious, and sometimes I can't help but think, what if Nezuko can never turn back into a human."

Turning a demon back into a human had no precedent.

Not only had Tanjirou never heard of it, but even among the Ubuyashiki family, the Rengoku family, and the entire history of the Demon Slayer Corps, there had never been a case where a demon turned back into a human.

Forget recorded history, even in the original story, the idea of a demon reverting to human form only appeared near the grand finale because of the human-transforming drug researched by Tamayo.

Although Tanjirou had a firm will and never-give-up spirit, whether he could actually turn his sister back into a human was still an unknown, and he lacked confidence in this regard.

Until today, this wish had finally come true.

The person who made all this happen was this very powerful, very mysterious, and extraordinary Great Teacher, whom he had not known for long.

Tanjirou had made up his mind.

"From now on, you're my big brother!"

"...No, let's not."

"No way! You are my big brother!"

"But I think..."

"Big brother!"

"..."

Tanjirou's loud voice literally left Reiger with no words to even reject him (in the literal sense).

It must be said, true to being a disciple of Rengoku Kyoujurou, he might not have mastered other lessons, but his loud voice and his ability to ignore advice were indeed learned to an exemplary degree.

"Fine, have it your way."

At that moment, Reiger simply let it be.

During their time on the Infinite Train, this guy had called him "big brother" for a while, and although this "big brother" was not that "big brother," it didn't matter— he'd put up with it.

"Your sister is okay now, right?"

Reiger asked.

"Yes, she is fine." Tanjirou's face softened as he said, "She just seems very tired, so I let her sleep. Tonight, I will ask Miss Shinobu to come over and check on Nezuko. I just hope she stays healthy."

"Okay," Reiger nodded and said, "So, you're not training today?"

"Yes, please allow me to rest for a day." Tanjirou said apologetically, "I'm worried about Nezuko and want to stay by her side, but once Nezuko's condition improves, I will definitely go!"

"Actually, it would be okay for you to quit now, wouldn't it?" Reiger suggested, "Your sister has turned back into a human, and there's no need for you to go after Kibutsuji Muzan anymore. With that being the case, taking your sister and going home away from the battlefield might not be a bad choice for you both."

Tanjirou fell silent all of a sudden.

"You only have Nezuko left as a family member, right?" Reiger said as he watched a troubled Tanjirou, "The same goes for her, as she only has you left. If you continue to fight and encounter powerful demons, leading to you unfortunately losing your life, then Nezuko would have no family to rely on."

"I..."

Tanjirou opened his mouth.

"You don't have to answer now," Reiger stopped him, "Think it over properly first. It's best to discuss it with your sister as well, there's no rush to give an answer."

Upon hearing this, Tanjirou closed his mouth, showing a complex expression.

"Think it through."

Reiger then turned and left.

Tanjirou did not speak again and simply bowed deeply once more toward Reiger's retreating figure.

He knew that Reiger was saying these things for his own good.

But he indeed wasn't ready to make a decision yet.

So, Tanjirou felt that he really needed to think carefully about what to do next.

......

Reiger had good reasons for saying those words to Tanjirou.

As the original protagonist, Tanjirou had many commendable qualities, and if cultivated well, he could definitely become an excellent combat force, even playing a critical role at crucial moments.

However, in the original storyline, Tanjirou's ending wasn't really good.

One of his arms had atrophied, he was blind in one eye, and due to activating the mark, his life would not extend beyond 25 years.

Although, for Tanjirou, this was a very happy ending since his sister turned back into a human and Kibutsuji Muzan was defeated, and he returned home with his friends to lead an ordinary life. He was certainly satisfied, but ultimately there were many regrets left behind.

Now, all of that had yet to happen.

The final battle hadn't begun, Tanjirou's mark hadn't awakened yet, and with himself involved, dealing with Kibutsuji Muzan wasn't an issue. Therefore, one more or one less Tanjirou didn't matter; it was unnecessary to let him wade into these muddy waters.

If Tanjirou were to truly die in battle this time, then his sister would indeed become utterly alone. Rather than that, it would be better to avoid the battlefield from the beginning.

But...

"After all, being the original protagonist, if it were so easy to give up, the villains across countless worlds would probably celebrate for days and nights."

Reiger somewhat anticipated what might happen next.

"Ah, I'm back."

"You're so slow, I've been waiting for you for so long."

"Hurry up, let's continue."

"Let's go!"

Unknowingly, Reiger had returned to the training ground, and the fully revigorated Pillars hurried him along.

Reiger gave a slight smile, drew his sword, and entered the training ground.

Before long, the flames and loud bangs returned, and the intense training resumed.