As crimson waves appeared in Riezel's hand, Tanjirou wasn't surprised.
After all, swordsmen who had mastered the breathing style could conjure fire, water, wind, and more. Emitting crimson light was no big deal and certainly not surprising.
Filled with anticipation, Tanjirou watched nervously as the crimson waves flowed from Riezel's hand to Nezuko's forehead, gradually covering her entire body.
Riezel controlled the decomposition magic with precision, careful not to make a mistake that could decompose a person, which would be a disaster. He targeted only the foreign substance inside Nezuko, Muzan's demon blood that didn't belong in a human body.
In the next moment, Nezuko suddenly began to convulse.
"Ugh..."
A slightly pained sound escaped from Nezuko's mouth, muffled by the bamboo tube she was biting.
Tanjirou almost couldn't resist rushing over, but he held himself back, choosing to trust Riezel.
Nezuko's shaking intensified, and the sounds she made grew more pained.
Eventually, her entire body began to emit smoke.
"Nezuko!"
Tanjirou finally couldn't hold back and called out, but unfortunately, neither Riezel nor Nezuko had the energy to respond to him at this moment.
Nezuko seemed to be enduring unbearable pain, causing her arms and legs to flail uncontrollably.
*Chi!*
Nezuko's sharp claws inadvertently scratched Riezel's hand, leaving a bloodstain on his arm.
Riezel frowned slightly but didn't stop, calmly continued to release the decomposition magic.
He had initially thought it would be simple—just decomposing Muzan's blood from Nezuko's body would restore her to human. Unfortunately, Muzan's blood had already merged with her flesh, completely transforming her body inside out. It wasn't something that could be decomposed so easily.
Riezel needed to find and decompose the 'inhuman parts' within Nezuko without harming other parts. Its difficulty was akin to plucking a thorn from an invisible cell, making one want to cry out that it was impossible.
In the past, Riezel indeed wouldn't have been able to accomplish this. Luckily, the current him had awakened the Transparent World, enabling him to see through bodies, observing the movement of every muscle and even each blood vessel.
As long as he concentrated a bit more, it wasn't impossible to extract the so-called 'thorn' embedded in an invisible cell.
In this manner, Riezel completely shut down all his senses, leaving only the minimum necessary, and focused intensely on observing every cell and every bit of flesh in Nezuko's body.
Before long, he extracted all the foreign substances he discovered inside, decomposing them with decomposition magic.
Clearly, this process was cumbersome, lengthy, and tested one's patience.
At least half an hour had passed, and Riezel was still at it.
Nezuko continued to struggle in apparent pain, emphasizing the difficulty of the task.
After half an hour, the smoke emanating from Nezuko began to lessen, and her pained sounds and struggles gradually eased as well.
About an hour later, as the crimson waves disappeared, Nezuko finally calmed down completely.
"Phew..."
Only then did Riezel let out a breath and stand up.
"How is she, Mr. Tsugikuni?"
Tanjirou, who hadn't blinked once during the entire process, kept his eyes glued to Riezel. Seeing that Riezel seemed to be done, he quickly asked.
However, Riezel didn't respond and merely gave Tanjirou a slight smile before turning to leave.
"Mr. Tsu—"
Tanjirou instinctively tried to call out, but in the next second, he was interrupted by another voice.
"Big... Brother..."
When that faint, drowsy voice echoed in the room and reached Tanjirou's ears, he felt as if struck by lightning, freezing in place.
After a moment of daze, tears welled up in Tanjirou's eyes once again.
===
Leaving Tanjirou's dormitory, Riezel immediately heard the sound of crying from inside as he stepped outside.
"Poor guy..." Riezel sighed, deciding not to rush off, and instead waited outside.
In the end, he ended up waiting for another half hour.
Finally, Tanjirou came out of the dormitory, wiping away his tears.
"Thank you, Mr. Tsugikuni, thank you so much!"
As soon as Tanjirou saw Riezel, he ran up to him without hesitation, bowing repeatedly.
"Alright, you've thanked me plenty of times already."
Riezel helped Tanjirou up, not letting him bow any further.
"No! No matter how many times I thank you, it won't be enough to express how I feel right now!" Tanjirou said with a tearful but relieved smile. "Nezuko is my only family left. I've always been worried, thinking about what I'd do if she never turned back into a human..."
A demon becoming human was unprecedented.
Not only had Tanjirou never heard of it, but even in the past records of the Ubuyashiki family, the Rengoku family, and the entire Demon Slayer Corps, there were no records of demons turning back into humans.
Not even in the original work was there any mention of demons becoming humans until Tamayo developed the cure for it.
Although Tanjirou had strong willpower and unwavering determination, he had always been uncertain about whether he could truly turn his younger sister back into a human, until today.
The one who made it possible was the very strong, very mysterious, and very powerful head instructor he had only recently met.
At this moment, Tanjirou made up his mind.
"From now on, you're my big brother!"
"...No, that's not necessary." Riezel raised his eyebrows.
"No way! You're my big brother now!"
"But I think—"
"Big Brother!"
"..."
Tanjirou's loud voice and determination left Riezel speechless, literally unable to refuse.
It had to be said, as Kyoujurou's Tsuguko, Tanjirou might not have mastered everything yet, but his loud voice and stubbornness had certainly surpassed his mentor's.
"Fine, do whatever you want."
Riezel didn't care anymore.
Back on the Mugen Train, this boy had also called him 'Big Brother' for a while. Even though it wasn't the same kind of 'Big Brother', it didn't really matter.
As such, he would just accept it.
"Is your little sister okay?" Riezel asked, changing the topic.
"Yeah, she's fine..."
A gentle expression appeared on Tanjirou's face as he answered.
"She still seems very tired, so I let her sleep. Tonight, I'll ask Miss Shinobu to come and check on Nezuko. I just hope she stays healthy."
"Alright." Riezel nodded. "So you're not practicing today?"
"Um, I'll be with Nezuko for now..." Tanjirou said apologetically. "I'm worried about her and want to stay by her side for a bit, but once she's better, I'll definitely come back!"
"Actually, you could just quit, you know?" Riezel suddenly suggested. "Your little sister's back to being human, so you don't need to look for Kibutsuji Muzan anymore. You could take her home and stay away from the battlefield. It wouldn't be a bad choice for both of you."
Upon hearing this, Tanjirou instantly fell silent.
"You're only left with Nezuko, right?" Riezel said. "And it's the same for her. If you keep fighting and end up running into a powerful demon and getting killed, Nezuko won't have any family left to rely on."
"I—"
"You don't have to answer now."
Tanjirou opened his mouth, wanting to say something, but Riezel interrupted him.
"Think it over carefully. Also, it's a good idea to talk it over with your little sister so you don't have to rush your decision."
Upon hearing this, Tanjirou closed his mouth, showing a complex expression.
"..."
Seeing Tanjirou's expression, Riezel immediately turned and left.
Seeing him leave, Tanjirou didn't say anything, only bowing deeply to Riezel's back.
He knew Riezel was saying those things for his own good.
Still, he really couldn't make a decision yet.
With this in mind, Tanjirou thought he should really take the time to consider what to do next.
===
In fact, Riezel saying those suggestions to Tanjirou for a reason.
As the protagonist, Tanjirou's body had many outstanding qualities. If developed properly, he would undoubtedly become an outstanding character, capable of playing a decisive role in critical moments.
However, in the original work's ending, Tanjirou's fate wasn't particularly a good one.
One of his arms had atrophied, one eye had gone blind, and due to using the Demon Slayer Mark, he wouldn't live past twenty-five.
Nevertheless, for Tanjirou, this was already a happy ending.
His younger sister had returned to being human, Muzan had been defeated, and he had returned home with his friends to live an ordinary life.
Even though Tanjirou was content with this outcome, it actually still left many regrets.
Fortunately, none of this had happened yet.
Since the final battle hadn't started, Tanjirou's Demon Slayer Mark hadn't awakened, and with himself around, Muzan wouldn't go unopposed.
Given this, Tanjirou didn't need to jump into this mess.
If Tanjirou were to die in battle this time, his younger sister would indeed be left all alone.
Knowing this, it would be better not to let him join the fight in the first place.
However—
'Well, he's the protagonist after all. If protagonists give up so easily, villains in countless worlds would probably cheer for three days and nights.'
Riezel somewhat anticipated what would happen next.
"Oh, you're back."
"You're so slow! I've been waiting for you forever."
"Hurry up, let's continue."
"Let's go!"
Unconsciously, instead of returning to his house, Riezel went back to the training ground, urged on by the fully re-energized Hashira.
Seeing them, Riezel smiled slightly, drew his sword, and entered the training ground.
Before long, the flames and explosions returned, and the hellish special training resumed.