Just when everyone thought Nezuko had merely regained her ability to speak, something strange began to happen.
Though she had always slept through the day and moved only at night, Nezuko suddenly began stepping out into the daylight — as though drawn to the sun itself.
Fortunately, Tanjiro noticed in time and stopped her.
But it was as if sunlight held an irresistible pull on her.
Whenever she found the slightest chance, she would try to run toward it.
This left Tanjiro deeply troubled.
In desperation, he tied a rope around both himself and Nezuko to keep her close.
But such a method could only work temporarily — Tanjiro couldn't keep watch over her every second.
He decided to seek Yosuke's advice.
When Tanjiro found him, Yosuke was in his laboratory.
Inside, along with Yosuke, were the Kocho sisters — Shinobu and Kanae — as well as Tamayo, and Kokushibō, who was currently undergoing examination.
Because Kokushibō and Yoriichi Tsugikuni were brothers by blood, their lineage was identical.
It was possible, then, that Kokushibō's ability to use the Sun Breathing was not due solely to Yoriichi's soul.
After all, the idea of a soul's lingering influence was far too abstract — neither the Kocho sisters nor Tamayo had ever witnessed such a thing.
They preferred to interpret Kokushibō's condition through scientific reasoning.
After running several tests on Kokushibō's blood, they managed to reach a tentative conclusion that seemed, at least, somewhat plausible.
Their theory was this:
The Divine Priest bloodline within Kokushibō's body temporarily suppressed his demonic nature and lifted his curse.
Since Moon Breathing was so closely related to Sun Breathing, this suppression allowed him to manifest the latter.
However, the result was far from satisfying.
They planned to conduct further research when Tanjiro arrived, visibly distressed.
"Yosuke-san," Tanjiro began, "Nezuko used to stay in her wooden box during the day. But ever since she regained her speech, she's been trying to go out into the sunlight."
Kokushibō, standing nearby, raised an eyebrow in surprise.
"Nezuko feels it too?" he asked.
Yosuke frowned slightly.
"Feels what?"
Kokushibō explained calmly,
"Ever since I freed myself from Muzan's curse, I've felt a strange longing for sunlight — a sense that being under it wouldn't harm me."
Yosuke crossed his arms, deep in thought.
"For demons, sunlight is death itself. Such a desire shouldn't exist. When facing a natural enemy, even animals instinctively flee — that's how survival instincts work."
Kokushibō nodded.
"Indeed. Our instincts as demons are far sharper than a human's. By all reason, I shouldn't feel any attraction to sunlight. And yet, when I extended my hand into its rays once before, my arm still burned instantly to ash."
Tamayo frowned.
"Instincts are unconscious responses when one encounters danger. If you truly feel drawn to sunlight, then either your instincts have failed — or sunlight genuinely no longer poses a threat to you."
Kokushibō turned to Yosuke.
"Speculation will lead us nowhere. Let's test it, Yosuke."
Yosuke immediately understood his intent.
He released his Demon King's aura, its oppressive force filling the room.
Kokushibō braced himself beneath it, then said,
"That's enough."
Yosuke withdrew his aura, and Kokushibō explained,
"My instincts are intact. Muzan's presence — and yours as well, Yosuke — exerts a natural dominance over those of us he created. Just now, my instincts urged me to flee. So, no — they haven't dulled."
Kanae rested her chin in her hand, murmuring thoughtfully,
"If his instincts are fine, then why would he still feel drawn to sunlight?"
Shinobu added her own question,
"And if he truly were immune to it, then why did his arm still burn when exposed?"
Everyone fell silent, lost in thought.
Finally, Kokushibō spoke.
"If we can't reason it out, then let me test it tomorrow myself."
"You can't!" Tanjiro exclaimed. "You already tried before — you'll be burned again!"
Kokushibō smiled gently.
"It's all right. If I can withstand sunlight, that means Nezuko can too. That would be a blessing for her."
The warmth in his voice, and the tenderness in his eyes toward Tanjiro and Nezuko, were unmistakable.
Tanjiro hesitated, guilt twisting in his chest. He couldn't shake the feeling that Kokushibō was doing this because of him.
Seeing his turmoil, Kokushibō added kindly,
"Besides… I wish to feel it once more — the sunlight I've long forgotten yet always longed for. That clear, boundless warmth…"
His mind made up, no one tried to stop him again.
Together, they waited for dawn.
When the first light of morning began to break, Tamayo suggested,
"Now. The sunlight is still weak — its damage minimal. If you're going to test it, this is the safest time."
Kokushibō nodded. Under the anxious gazes of everyone present, he carefully extended one arm into the sunlight.
The moment the rays touched his skin, a faint sizzling filled the air, followed by the acrid scent of burning flesh.
Yosuke saw the arm blacken completely and called out,
"Kokushibō, that's enough!"
But Kokushibō didn't withdraw.
The sun had now fully risen, its light spilling across his entire body.
Gritting his teeth, he said through the pain,
"No… this time, I can feel it — something inside me is changing!"
And with that, he stepped fully into the sunlight.
Under the full blaze of day, his agonized cries echoed through the Butterfly Mansion.
Everyone panicked and rushed forward to pull him back, but Kokushibō stopped them with a shout.
"I can still endure it!"
His entire body was being scorched, his skin blistering and melting away.
Yosuke noticed faint traces of his body beginning to disintegrate.
"Kanae! Shinobu! Pull him back, now!" Yosuke ordered urgently.
Just as the sisters were about to move, a miracle occurred.
The ashen flakes rising from Kokushibō's body stopped.
Before their eyes, the burns on his skin began to heal — rapidly, visibly — until his body was completely restored.
The impossible had happened.
Kokushibō had conquered the demon's greatest weakness — sunlight itself.
With his eyes half-closed, he stood quietly beneath the radiant sky, bathed in warmth.
His voice trembled as he whispered,
"After hundreds of years… I can finally feel it again — the bright, gentle sunlight."