Kokushibō's success in overcoming sunlight left everyone utterly speechless.
For a thousand years, Muzan Kibutsuji had pursued that very goal — to walk beneath the sun — and yet, it was one of his own creations who achieved it so easily.
How ironic it was that the first being to become the ultimate lifeform turned out to be a demon born of Muzan himself.
Having witnessed Kokushibō's transformation, Zenitsu could hardly contain his excitement.
"Tanjiro! Quickly, let Nezuko try it too! If she can overcome sunlight, she'll be able to live like a normal person again!"
Tanjiro, overwhelmed by hope, was about to agree — but Kokushibō stopped him immediately.
"No."
Tanjiro and Zenitsu both looked at him in confusion.
"Why not?" Tanjiro asked.
Kokushibō's expression was grave.
"When I stepped into the sunlight just now, I was a breath away from turning to ash. If I hadn't endured at that exact moment, I would have died. Unless we can be absolutely certain Nezuko will survive it, we must not take that risk."
Yosuke nodded in agreement, his tone equally serious.
"Kokushibō is right. A miracle has already happened once — we can't rely on another. What we must do now is turn that miracle into a guaranteed outcome."
The Kocho sisters and Tamayo immediately began a thorough examination of Kokushibō's body. What they discovered shocked them.
His blood composition had completely changed.
Originally, over seventy percent of Kokushibō's blood had carried Muzan Kibutsuji's demonic essence.
Now, after conquering sunlight, the concentration of that "Demon King's blood" had dropped drastically. Muzan's blood characteristics had vanished entirely — replaced by something new, something unique.
Moreover, Kokushibō no longer craved human flesh or blood. In other words, by overcoming sunlight, he had ceased to be a demon altogether. He had become something beyond both human and demon — a new kind of being.
Further analysis revealed something extraordinary: Kokushibō's blood held tremendous potential for the humanization medicine the researchers had been working on.
Up to that point, Tamayo and the Kocho sisters' efforts to create such a medicine had hit a dead end. No matter what they tried, the results remained unchanged.
But now, using Kokushibō's blood as the main ingredient, they worked tirelessly, day and night, until they finally created a drug that could, in theory, allow demons to withstand sunlight.
To test its effects, Tamayo and the Kocho sisters injected the serum into Gyutaro, who had been chosen as a test subject.
The results, however, were tragic.
Instead of resisting sunlight, Gyutaro's body violently rejected the drug — the demon blood within him was completely destroyed, and he died on the spot.
Their first trial had failed — and yet, it wasn't a complete failure.
After repeated analysis, they discovered something crucial:
When they added blood from Amane Ubuyashiki, a member of the sacred Divine Priest Clan, the destructive effect of the drug vanished.
Injected into a demon, this version of the serum no longer obliterated the demon blood.
However, once the Divine Priest blood lost its potency, the medicine's destructive properties resurfaced — and the test subjects again perished.
Thus, they concluded that the serum could only be safely used by Nezuko — because, like Kokushibō, she was born with the Divine Priest bloodline within her.
At last, the perfected medicine was ready.
Tanjiro held the small syringe, his hands trembling, his heart overflowing with emotion.
Looking tenderly at his sister, he said softly,
"Nezuko… you'll soon be able to stand in the sunlight. A new life is waiting for you."
Filled with hope, Tanjiro carefully injected the medicine into Nezuko's body.
At once, her small, childlike form began to grow — her body expanding rapidly to its normal size. Then her features changed again, her appearance shifting into a half-demon state.
A small horn sprouted from her forehead, and delicate willow-like markings spread across her limbs.
The transformation coursing through her body brought unimaginable pain.
Nezuko's screams echoed through the room.
Tanjiro, terrified, caught her as she writhed on the floor and turned anxiously to Tamayo.
"What's happening to her?"
Tamayo explained calmly, though her eyes were filled with sympathy.
"The serum is rewriting her blood entirely. The pain is inevitable."
Nezuko's transformation deepened — her body now that of a grown woman in her twenties, her horn fully formed, her body covered in flowing demon patterns.
Overwhelmed by agony, her instincts took over.
Consumed by rage and pain, she lost all sense of self and lunged at Tanjiro, her sharp fangs bared, aiming for his shoulder.
Yosuke reacted instantly, thrusting his arm between them.
Nezuko bit down hard, her fangs sinking deep into his flesh. Blood spilled, dripping down onto Tanjiro's clothes.
"Yosuke-san!" Tanjiro cried, panic rising.
Yosuke gritted his teeth and shook his head.
"Don't worry about me — just hold on! She'll pull through soon!"
As Nezuko's screams filled the air, Tanjiro began to sing — the lullaby their mother used to sing to them when they were children.
"Little bunny on the mountain, why are your ears so long?
When you were small, your mama ate long leaves, that's why your ears grew so long…"
Whether it was the effect of the song or the medicine itself, Nezuko suddenly froze.
Her jaw slackened, releasing Yosuke's arm. Her eyes went unfocused, staring into the distance.
In her mind, she heard her mother's gentle voice — Kie Kamado — singing that same lullaby.
Nezuko remembered her childhood: walking through the mountains hand in hand with her mother, as that familiar song drifted through the air.
"Little bunny on the mountain, why are your eyes so red?
When you were small, your mama ate red berries, that's why your eyes are so red…"
Young Nezuko's voice echoed in her memories.
"Mama, are Tanjiro's eyes red too because you ate red berries when he was in your tummy?"
Kie Kamado didn't answer. She only smiled warmly and said,
"Nezuko, it's time to wake up now."
Tears welled up in Nezuko's eyes. She began to sob uncontrollably.
The horn on her head crumbled and fell away.
The willow markings faded from her skin.
Her body returned to its normal human form.
Exhausted, she fell asleep in Tanjiro's arms, her cheeks still wet with tears.
"Nezuko…" Tanjiro whispered, his voice thick with emotion, looking down at his sister with boundless tenderness.
Tamayo carefully drew a small vial of blood from the sleeping Nezuko and ran her tests.
When the results came in, everyone exhaled in relief.
Nezuko's blood composition was now identical to Kokushibō's.
In other words —
Nezuko, too, had overcome the sun.