In order to gain new strength, Tokitō Muichirō temporarily stayed at the Butterfly Mansion to train under Kokushibō Tsugikuni, learning a new Breathing Style — the Moon Breathing.
As expected of the prodigy who had become a Hashira in just two months, Muichirō's progress in mastering the Moon Breathing was remarkably swift.
It was as though the style itself had been created for him — within only a month, he had mastered all of its forms.
Before anyone realized it, half a year had passed.
During that time, aside from a few scattered demon attacks, there had been no sign of Kibutsuji Muzan or the Twelve Kizuki.
They had seemingly vanished without a trace.
On the roof, Inosuke lay sprawled out lazily under the sun, chewing on a piece of grass and muttering with boredom,
"Ugh, this is so boring! Not even a single demon to fight. My whole body's going stiff from doing nothing~"
Tanjiro smiled gently.
"Isn't that a good thing? The fact that we can relax like this means the world outside is peaceful."
Zenitsu nodded in agreement.
"Tanjiro's right. Things are great right now! We don't have to worry about dying heroically at any moment, and I get to spend all this time with Nezuko, deepening our bond~"
Hearing that, Tanjiro sighed helplessly.
"Zenitsu, stop teaching Nezuko those weird things! Yōsuke-san has scolded you several times already. Haven't you learned your lesson yet?"
Zenitsu puffed up indignantly, completely lacking self-awareness.
"What do you mean, weird things?! Everything I teach Nezuko is a pure expression of my love for her~!"
Tanjiro tried to reason with him patiently.
"Zenitsu, Nezuko is still in a state of amnesia right now. There's a lot she doesn't understand.
If you really want to be with her, you need to become reliable first. If you keep acting spoiled, she'll just treat you like a little brother."
Zenitsu thought back to all the times he had tried to flirt with Nezuko, only for her to smile and pat him on the head like a child.
Wait... had she really been treating him like a younger brother this whole time?
Tanjiro, being both Nezuko's brother and Zenitsu's close friend, knew well how deeply Zenitsu felt about her — and worried about him constantly.
"Zenitsu," Tanjiro advised seriously, "you need to show Nezuko your dependable, manly side.
Let her see that you're strong — that she can count on you. And when the time is right, you have to tell her how you feel. Make sure she knows."
Zenitsu's eyes lit up with determination.
"Tanjiro — teach me!"
Tanjiro flinched in surprise.
"M-Me? I don't know! You can't just ask me that out of nowhere — I don't have any experience either!"
Zenitsu suddenly dropped into a full dogeza bow, his forehead hitting the roof tiles with a thud.
"You're Nezuko's brother! You must know something! Tanjiro, you're my best friend — my future brother-in-law! My happiness is in your hands!"
Tanjiro rested his chin on his hand, deep in thought, trying to come up with advice.
He wanted to help Zenitsu... but instead, his mind kept filling with images of Kanao Tsuyuri.
No matter how hard he tried, her face wouldn't fade from his thoughts.
Tanjiro's cheeks slowly turned red, and steam practically began to rise from his head like a chimney.
Zenitsu blinked in confusion and looked up at the sky.
The weather was pleasant, but not hot enough for that reaction.
"Tanjiro, what's up with you? Is it hot today?"
"N-no, not at all!" Tanjiro waved his hands frantically, trying to hide his embarrassment.
He coughed to compose himself and continued,
"Anyway, Zenitsu, from now on you should try to show your cool side in daily life.
Stop being lazy. Stop complaining all the time—"
But before he could finish, Zenitsu started frantically winking at him.
"Huh? Zenitsu, what's wrong with you?" Tanjiro asked in confusion.
A calm, slightly chilling voice suddenly came from behind him.
"Tanjiro... you seem very experienced when it comes to pursuing girls?"
Tanjiro froze.
He turned his head stiffly — and there stood Kanao.
Though her face was expressionless, Tanjiro could feel a cold wind passing right through him.
"W-wait, Kanao, it's not like that! Let me explain—" he stammered.
Kanao tilted her head slightly.
"Why are you explaining it to me? I only came here to tell you that Nezuko has been looking for you ever since she woke up."
With that, she turned and disappeared as quietly as she had appeared.
The moment Tanjiro heard Nezuko was awake, he too vanished from the rooftop in a rush.
Inosuke blinked, confused.
"What's up with Santarō? He's acting weird. Is it because of... Shōtarō?"
Zenitsu groaned and corrected him.
"It's Tanjiro and Kanao, not whatever you just said! You wouldn't get it, Inosuke — you're too straightforward.
Looks like Tanjiro's springtime of love has come.
When will my spring ever arrive...?"
Meanwhile, in the research laboratory, the Butterfly sisters were haggard, dark circles heavy under their eyes as they carefully shook a test tube of liquid.
Even Tamayo — though a demon who technically shouldn't feel fatigue — looked utterly exhausted, jotting down data on sheets of paper.
For nearly a week straight, Tamayo and the Kocho sisters had been working without rest.
The development of the Human Transformation Drug had reached its final stage — only one last breakthrough remained.
No one dared stop, afraid that taking even a short break might make them lose their inspiration.
Under Kanae's steady hands, the liquid in the test tube became increasingly cloudy.
Her voice was tense.
"It's time. Shinobu — add the drop."
Shinobu carefully dripped a small amount of crimson liquid into the tube —
a concentrated extract purified from the combined blood of Kokushibō and Nezuko.
The moment the red liquid touched the cloudy solution, the three women held their breath, eyes locked on the reaction before them.
Slowly, the murky mixture began to clear...
and finally became as transparent as pure water.
For a moment, silence filled the room — and then, they all exhaled in relief.
Gripping each other's hands, they could barely contain their joy.
"We did it... the Human Transformation Drug is complete!"
But it was not yet time to relax.
Every new medicine required testing — only a successful trial could prove its effectiveness.
On the bed behind Tamayo lay a man tightly restrained with ropes.
The other test subjects, demons captured earlier for research, had already perished.
This man had been newly captured by Yōsuke-san and brought back for this final experiment.
Kanae filled a syringe with the completed drug and injected it into the man's arm.
At first, the bound demon thrashed wildly.
But soon after the injection, his movements slowed... and then stopped entirely.
Before their eyes, the crimson hue of his pupils faded, returning to a normal human color.
The horns on his head crumbled away and fell.
From outward appearance alone, he was now no different from a human being.