Since Rengoku Shinjuro himself had raised the matter, Kokushibo saw no reason to stand on ceremony.
"I would like to learn the Breathing Technique you mentioned," he said calmly. "I wish to see whether my swordsmanship can be refined further."
Shinjuro fell silent.
Breathing Styles were not entirely forbidden knowledge, yet they were not something to be taught lightly. Typically, such techniques were passed down within the Demon Slayer Corps, or through trusted disciples who had proven both their character and resolve.
And yet
Iwakatsu was neither a member of the Corps nor someone who could formally join in his current condition.
That made things… complicated.
Kokushibo noticed the hesitation but did not press the matter. There was no need.
He would agree.
That much, Kokushibo was certain of.
After all, the importance of Shinjuro's wife was something even an outsider could perceive. A man like him fierce, straightforward, yet deeply devoted would not allow hope to slip away so easily.
The room fell quiet.
At that moment, Kokushibo's attention shifted.
Light footsteps.
A child entered, carefully carrying a bowl of water into the adjacent room the same room from which the sound of coughing had been echoing. After placing it down, the child quietly withdrew.
Kokushibo tilted his head slightly.
"…I hear someone. Who is that?"
Shinjuro glanced over, then called out.
"Kyojuro, come here!"
The child returned promptly. Though still young, he carried himself with remarkable composure.
"This is my friend, Iwakatsu," Shinjuro said. "Show your manners."
The boy bowed respectfully.
"Greetings, Uncle Iwakatsu."
Kokushibo raised a hand slightly in acknowledgment.
"…A well-mannered child."
This was none other than Kyojuro Rengoku in his youth.
After straightening, Kyojuro leaned toward his father and whispered urgently. Though his voice was hushed, Kokushibo's heightened senses caught every word.
"Father… Mother's condition has worsened. She… coughed up blood again."
Shinjuro froze.
In that instant, all hesitation vanished.
He turned back toward Kokushibo, his expression resolute.
"I will teach you Flame Breathing."
Kokushibo's lips curved faintly.
Internally, satisfaction stirred but outwardly, he maintained restraint.
"…There is no need to force yourself," he replied. "I would not wish to place you in a difficult position."
But Shinjuro continued, his tone firm.
"However, I have one condition."
Both Kokushibo and Kyojuro looked at him.
"I want you to take Kyojuro as your godson."
For a moment, silence fell.
Kyojuro blinked.
Kokushibo… paused.
…What?
Even he had not anticipated such a proposal.
Shinjuro, however, had already made his decision.
He was not a fool.
Allowing an outsider to learn Flame Breathing carried risk. But Kokushibo's demeanor, his conduct, his willingness to help these spoke of a man guided by principle.
By tying him to the Rengoku family through Kyojuro, Shinjuro created a bond one that would serve as both trust and restraint.
In essence, he was staking his honor on this man.
Kokushibo understood.
Perhaps not entirely but enough.
Seeing his son's confusion, Shinjuro spoke plainly.
"Iwakatsu knows a physician one capable of saving your mother."
Kyojuro's eyes widened.
Understanding dawned instantly.
Without hesitation, the boy knelt before Kokushibo.
"Please… accept me as your godson!"
His voice trembled not with fear, but with hope.
He was still a child.
And children, when faced with the chance to save someone they loved, could cast aside pride and composure without a second thought.
There was nothing shameful in that.
Kokushibo looked down at him.
This outcome had not been part of his plan.
Yet
He placed a hand gently upon Kyojuro's head.
"…Very well."
His voice softened, though it retained its usual calm authority.
"I will bring that physician."
A promise.
Simple, yet absolute.
Sensing that the hour had grown late, Kokushibo rose to leave.
Both Shinjuro and Kyojuro urged him to stay the night, but he declined without hesitation.
Remaining until dawn would be… inconvenient.
"…The physician resides far from here," he said. "It may take time to bring her."
He paused briefly before adding,
"If her coughing worsens, have her consume pears. It may provide some relief."
With that, he turned and departed, his figure gradually swallowed by the night.
The Rengoku father and son stood in silence, watching until he vanished completely from sight.
Inside the bedroom, Shinjuro approached the futon.
There lay Ruka Rengoku, propped weakly against her pillows. A cloth stained with fresh blood rested beside her.
The sight tightened his chest.
He could endure battle.
He could endure pain.
But this watching her suffer was unbearable.
"Ruka," he said softly, kneeling beside her. "I met someone… He knows a physician of extraordinary skill. He will bring her here."
Ruka had already overheard much of the conversation.
Her expression remained calm, though faintly troubled.
"You are placing a great deal of trust in a stranger," she said quietly. "What if he is not what he seems?"
She did not finish the thought.
As the wife of a Hashira, caution came naturally.
Shinjuro reached out, gently holding her hand.
"…Even so, I must try."
His voice wavered, just slightly.
"Promise me you will accept the treatment."
After a moment, her resistance softened.
"…Very well."
Elsewhere, in another room, Kyojuro sat awake.
Beside him, his younger brother, Senjuro Rengoku, stirred and sat up sleepily.
"Brother… what happened?"
Kyojuro pulled him into an embrace, his eyes shining with renewed hope.
"…Mother will be saved."
Neither of them could possibly imagine
That the physician Kokushibo intended to bring was a demon.
And yet, fate had already chosen its path.
At this moment, only Tamayo possessed the knowledge to treat such an illness.
As Kokushibo walked alone beneath the night sky, his thoughts deepened.
He could not act recklessly.
Not yet.
The flow of events what one might call the "main course" of fate had not begun.
If he interfered too early, the balance could shift unpredictably, leading to consequences even he could not foresee.
What did he truly possess at this moment?
The power of an Upper Rank One.
Mastery of Moon Breathing.
The Transparent World.
The Demon Slayer Mark.
And yet
None of these granted him certainty over the future.
In truth…
He still stood at the edge of the unknown.
Then
A sudden chill ran through him.
A presence.
A command.
His body tensed.
"…Muzan."
The will of Muzan Kibutsuji had reached him.
Cold sweat formed along his skin.
Had he been discovered?
Had his proximity to the Demon Slayer Corps drawn suspicion?
Before he could think further
The ground beneath him shifted.
A doorway opened.
He fell.
Darkness swallowed him whole.
When he emerged, he stood within the shifting halls of the Infinity Castle.
The domain of Infinity Castle.
Kokushibo immediately lowered himself to one knee.
"Greetings, Lord Muzan."
A voice sounded behind him calm, yet absolute.
"Rise."
Kokushibo obeyed.
Moments later, Muzan stood before him.
His presence alone was suffocating.
"Kokushibo," Muzan said slowly, his crimson eyes gleaming faintly, "I have a task for you."