As the battle in the Infinity Castle raged on, some were unlucky and got separated, while others were fortunate enough to be grouped together and acting together.
Tanjirou was one of the lucky few.
When Tanjirou was transported to the Infinity Castle, he found himself alongside two familiar faces he interacted with daily—Zenitsu and Inosuke.
Surprisingly, the atmosphere between the three was not as harmonious as it usually was.
On any other day, Inosuke would be recklessly charging ahead, with Tanjirou and Zenitsu scrambling behind to catch up and stop him. But today, it wasn't Inosuke charging ahead without a thought—it was Zenitsu.
Surprisingly, this normally timid and fearful youth, who avoided missions and dreaded encountering demons for fear of danger, was now marching forward in silence, his face cold and unyielding.
Tanjirou and Inosuke followed behind Zenitsu, walking and talking.
"Hey, sleepyhead, what's up with you?"
Inosuke, always blunt, asked Zenitsu directly.
"You've been acting strange lately. Did you eat something bad?"
Hearing this, Tanjirou could deeply relate to Inosuke's feelings.
Zenitsu had indeed been acting very strange recently.
Since arriving at the Swordsmith Village and receiving a letter that changed his complexion drastically, Zenitsu had stopped being his usual timid, tearful self. He became silent, disciplined, and trained relentlessly, completely unlike his old self.
In the past, the training at the Swordsmith Village would have had Zenitsu complaining within three days, whining by the fifth, and practically giving up by the seventh, acting like he was on the brink of dying.
Yet this time, Zenitsu neither complained nor shirked his training as if an obsession drove him. He trained harder than both Tanjirou and Inosuke, surprising many who knew and understood him.
During this month, Zenitsu only smiled once in front of everyone, which was the day he learned Nezuko had turned back into a human, but after that, he never smiled again.
Zenitsu trained as if his life depended on it, a sight that worried Tanjirou deeply.
Inosuke, despite his seemingly carefree and boisterous nature, was more sensitive and perceptive than most, particularly about Zenitsu's change. Due to this, today, he didn't charge around yelling as usual but just quietly followed Zenitsu.
Unfortunately, Zenitsu didn't seem to appreciate it.
"Stop being so noisy..." Zenitsu said without looking back, continuing to walk forward with a cold expression. "You're so loud, I can't hear anything. We won't be able to find that guy if you don't quiet down."
"So, who exactly are you looking for?" Inosuke insisted. "Did that guy make you mad or something? Tell me and I'll cut him down for you!"
"No need." Zenitsu replied coldly, his voice low. "He can only be killed by my hands."
"You—"
Inosuke seemed like he wanted to say something more, but Tanjirou stopped him.
"Alright, Inosuke, calm down, don't get worked up."
Tanjirou soothed Inosuke before turning to Zenitsu.
"What happened, Zenitsu? Tell us."
"..."
Zenitsu remained silent, not saying a word.
Seeing him like this, Tanjirou showed a worried expression.
"Aren't we friends?" Tanjirou continued. "Nezuko is also worried about you, y'know?"
Hearing these words, Zenitsu finally stopped walking.
Tanjirou and Inosuke stopped as well, watching Zenitsu silently, their concern evident in their eyes.
Seeing Tanjirou and Inosuke like this, Zenitsu clenched his fists and then sighed.
"You probably don't know this..." Zenitsu said softly. "I actually have a senior brother."
""A senior brother?""
Looking at each other, Tanjirou and Inosuke were taken aback.
"My master used to be the Thunder Hashira of the Demon Slayer Corps."
Zenitsu told Tanjirou and Inosuke the truth.
In the Demon Slayer Corps, swordsmen who excelled in Thunder Breathing and were promoted to Hashira using Thunder Breathing were called 'Thunder Hashira'.
Zenitsu's master was the previous Thunder Hashira, a Hashira-ranked swordsman who had mastered Thunder Breathing to its peak.
After his master retired, there has been no Thunder Hashira in the Demon Slayer Corps anymore, leaving Thunder Breathing without a Hashira among the five fundamental breathing styles to this day.
Truthfully, this was inevitable.
Thunder Breathing was known to be one of the hardest breathing styles to practice. Due to its extreme speed and explosive power, it demanded very high physical ability, making its practice almost as difficult as Sun Breathing.
In the history of the Demon Slayer Corps, among the five fundamental breathing styles, Flame Breathing and Water Breathing always had a Hashira, never missing. On the other hand, the other three breathing styles had periods of absence, with Thunder Breathing being the most absent, highlighting its difficulty.
Zenitsu's master was the previous Thunder Hashira, and his ability to become a Hashira with Thunder Breathing demonstrated his immense strength.
However, Zenitsu mentioning his master clearly wasn't to boast about his strength.
"I was deceived and fell into heavy debt, and it was my master who spent his money to buy me and took me in as his disciple, bringing me to the mountains to practice."
Zenitsu began to tell his past.
"My master, Grandpa was a very strict person... Because Thunder Breathing was so difficult to practice, he always trained his disciples harshly."
"Because the training was so tough, and mastering Thunder Breathing was insanely hard, I always tried to avoid training and run away. Even so... Grandpa never gave up on me and always brought me back, time and time again."
"In the end, I only learned the first form of Thunder Breathing... Even now, I haven't mastered any of the other forms."
Thunder Breathing had a total of six sword forms, which was one of the fewest among the many breathing styles.
Even so, no one thought Thunder Breathing was weak.
On the contrary, Thunder Breathing was so strong that it only needed six forms, which were enough to slay demons, leaving them with no escape.
However, Zenitsu had only learned the first form and none of the other five forms.
"Despite that, Grandpa never blamed me... He even told me that if I could only learn this one sword form, then I should master it to the peak, to the extreme of extremes."
When talking about the master he called 'Grandpa', Zenitsu's expression was no longer cold.
Until—
"But, that guy... that guy was different from me."
As Zenitsu mentioned that 'guy', he gritted his teeth.
"He had more talent than I did, was more skilled, and even started training earlier. Even the notoriously difficult Thunder Breathing, he learned five of its sword forms."
"If he kept training and mastered the last form, it wasn't out of the question for him to take Grandpa's place and become the Thunder Hashira the Demon Slayer Corps had been missing for so long."
"I could never hold my head up in front of him, so I always thought that if someone were to become Thunder Hashira, it should be him."
"But—"
"But you see, humans have their limits, so I chose to become a demon, Zenitsu!"
Zenitsu's voice, filled with hatred and anger, was suddenly interrupted by someone.
As this stranger's voice rang out, Tanjirou, Zenitsu, and Inosuke all quickly turned around, placing their hands on the hilts of their swords.
In front of them, a youth slowly walked around the corner and came into view.
He had black hair and green eyes, thick eyebrows, and wore a magatama necklace and bracelet. He carried a sword, had sharp ears, deathly pale skin, black tiger-like stripes on his face, and fangs that protruded from his mouth as he grinned.
Without a doubt, he was a demon.
However, he was clearly not just any demon, as evidenced by the Japanese characters in his eyes.
Nonetheless, Zenitsu didn't notice this, as the moment he saw this youth, his breathing became heavy.
"KAIGAKU—!!!" Zenitsu shouted angrily.
"Yo, long time no see, Zenitsu."
Kaigaku greeted Zenitsu with neither enthusiasm nor friendliness. Instead, there was a sense of aggression.
"So, you made it into the Infinity Castle too, huh? Good, good..." Kaigaku said with a smile. "After Lord Muzan ordered the complete annihilation of the Demon Slayer Corps, I thought, in that case, I should be the one to take care of you."
"Luckily, we met smoothly." Kaigaku's smile turned sinister, full of sarcasm. "Otherwise, a weakling like you would have surely been taken out by those little demons the moment you arrived here."
His words didn't make Zenitsu lose his temper, but they did make the impulsive Inosuke draw his two swords.
"You damn bastard!" Inosuke exclaimed angrily.
"Wait, Inosuke!"
Tanjirou hurriedly stopped Inosuke.
"Something's not right, look at his eyes!"
His words finally made Zenitsu and Inosuke notice the Japanese characters in Kaigaku's eyes.
One eye had 'Upper Rank', while the other had 'Seven'.
""Upper Rank... Seven...?""
Tanjirou, Zenitsu, and Inosuke were shocked.
What happened?
Weren't there only six Upper Ranks?
How did an Upper Seven appear?
"Oh? You mean this?"
Kaigaku pointed to his eyes, his smile not fading.
"That's right, I am Upper Seven—the first demon in history to hold this title, to be exact."
"Well, the Twelve Kizuki were officially disbanded when the Lower Ranks were dismissed, so Lord Muzan recently reorganized the Kizuki."
"Since he doesn't need the useless Lower Ranks, there are no more Lower Rank positions in the Kizuki, but an additional seat was added to the Upper Ranks."
"From now on, there are no more Twelve Kizuki, only us seven Upper Ranks—the new Kizuki."