Morning light streamed through the skylight of the Butterfly Mansion laboratory, scattering across the rows of bottles and jars, breaking into tiny, glimmering fragments.
Tamayo stood by the doorway, her gaze filled with worry as she watched the figure bent over the workbench.
Kocho Shinobu wore a white lab coat with the sleeves rolled up, several gauze patches covering her forearms—small cuts from the glass shards during yesterday's experiment. The dark circles beneath her eyes had deepened since the day before; clearly, she hadn't slept all night.
She held a dropper in her trembling fingers, carefully releasing drops of pale violet liquid into a test tube—her hand shaking so slightly that even she didn't notice.
"Shinobu, rest for a bit."
Tamayo quietly stepped inside and set a cup of warm herbal tea by her side. "You've been working for three straight days. A human body can't handle that kind of strain."
The dropper paused midair. Shinobu lifted her head and gave her usual gentle smile, but exhaustion hid beneath it. "Tamayo-san, just a little longer. I'm close to finding the ideal concentration of the wisteria extract. If I adjust it just a bit more, this toxin will completely destroy Muzan's cell activity. When that happens, we'll have a real chance against him."
Her voice was hoarse, almost imperceptibly so. As she rubbed her eyes, she accidentally smeared a bit of the medicine on her cheek, looking slightly disheveled.
Seeing her like that made Tamayo's heart ache like a needle prick. Ever since Oyakata-sama had confirmed Tomioka Giyu's death, Shinobu had changed—burying every emotion deep inside and numbing herself with endless work.
Tamayo didn't need rest; she was a demon. But Shinobu was human, and her body would collapse if this continued.
"Shinobu," Tamayo said seriously, sitting down beside her, "this isn't the way. If Tomioka-san knew you were hurting yourself like this for him, he wouldn't be happy."
At the sound of his name, Shinobu's smile froze. Her grip on the dropper tightened, and a few drops of violet liquid spilled onto the table, leaving faint stains.
She quickly wiped them away with a tissue, her voice barely audible. "I'm fine, Tamayo-san. I just want to finish the antidote as soon as possible..."
Tamayo sighed softly. She couldn't bear to see her like this any longer.
"Tomioka-san might still be alive."
The words left her lips quietly, but in the small laboratory, they struck like thunder.
Shinobu froze completely. Slowly, she lifted her head, eyes wide, staring at Tamayo. Her lips parted, but no sound came out.
After a long silence, her trembling voice finally emerged. "Tamayo-san, what... what did you say? Tomioka-san... is still alive?"
Tamayo nodded, her tone calm yet heavy. "Oyakata-sama received a report from Kamado Tanjiro and Tō. The mysterious proprietor of the Entertainment District—'Mushiki'—his appearance and mannerisms match Giyu almost exactly. Tanjiro also sensed faint traces of demon scent on him. Oyakata-sama believes that Giyu may have been captured by an Upper Moon during the battle at Kirigakure Village, turned into a demon against his will, and now remains under Muzan's control—unable to reveal himself to us."
"Turned... into a demon..."
Shinobu repeated the words softly. Then, without warning, tears spilled from her eyes, falling onto the workbench and spreading into small, shimmering puddles.
The strength she had held onto so long crumbled all at once. She covered her face with her hands, shoulders trembling, no sobs escaping—only the sound of muffled, stifled weeping through her fingers.
She was both relieved—thankful that Giyu hadn't died, thankful he was still alive—and terrified. Terrified that he had become a demon, that he was being controlled by Muzan, that every day he might be trapped in agony.
That quiet, stoic man who always showed his care through small acts. The one who would share his rice ball with a stray cat. How could that Giyu ever become a demon?
Tamayo said nothing. She only reached out to gently pat Shinobu's back, offering silent comfort. She knew Shinobu needed this—to release all the grief she had buried for months.
It took a long time before Shinobu finally calmed down. She wiped her tears, and light flickered again in her eyes. "Tamayo-san, if Giyu has become a demon, can we create a drug that turns demons back into humans?"
"In theory... it might be possible."
Tamayo hesitated for a moment, then spoke slowly. "A demon's body is reconstructed by Muzan's blood. If we could develop a formula that completely removes Muzan's cells while simultaneously repairing the damaged human body, it might be able to restore a demon to human form. And since Giyu can use Sun Breathing, that power could suppress Muzan's cells, greatly increasing the chance of success."
"Really possible?"
Shinobu's eyes lit up. She straightened, grabbed her pen, and began jotting notes rapidly in her notebook.
"What materials do we need then? The wisteria flower extract, of course, and the inhibitor we found for Muzan's cells... maybe we'll also need a Sun Breathing energy carrier..."
Her words came faster and faster, her eyes bright with excitement, all traces of exhaustion gone.
Watching her, Tamayo felt a quiet sense of relief. Shinobu had finally found a new goal—not just burying herself in endless work to numb her pain, but researching with purpose and hope. That was the Kocho Shinobu she knew.
In the following days, Shinobu remained in the lab as before, but her condition had completely changed. She now ate on time, rested for an hour or two when Tamayo insisted, and her smiles had grown more genuine.
Together, they combed through ancient texts, searching for records on "human reconstruction" and "cell purification." They spent each day adjusting formulas again and again. Even though they failed countless times, neither gave up.
Tamayo often watched Shinobu's focused expression and felt a faint guilt stir inside her. She had told Shinobu about Giyu's transformation into a demon on her own accord. Although she meant well, it violated Oyakata-sama's command to keep it top secret.
Shinobu was the person Giyu cared about most. She deserved to know the truth—but even so, Tamayo knew she needed to report and apologize.
That evening, while Shinobu was organizing experiment data, Tamayo quietly slipped out of the Butterfly Mansion and made her way to Oyakata-sama's residence.
Ubuyashiki Kagaya was sitting in the courtyard, gazing at the glowing sunset. When he heard her footsteps, he slowly turned, a gentle smile on his face. "Tamayo-san, you've come."
Tamayo froze for a moment before bowing deeply. Her tone carried apology. "Oyakata-sama, forgive me. I revealed the truth about Tomioka Giyu's transformation to Shinobu without permission. I've come to accept any punishment."
"You don't need to apologize."
Kagaya shook his head softly, his voice calm. "Shinobu is someone Giyu cares for deeply. She has the right to know the truth. And because of that news, she's regained her sense of direction and a new purpose—to create the 'human restoration medicine.' For us, that's a good thing."
He paused briefly, then continued, "However, this matter must remain secret. It cannot be allowed to spread. Both your and Shinobu's research must be carried out with caution. Muzan cannot be allowed to sense anything unusual. If we can truly create this 'human restoration medicine,' it will not only save Giyu but also countless demons who were forced into their fate by Muzan. Its value to our mission—to destroy Muzan—is beyond measure."
Tamayo lifted her gaze to meet his gentle eyes. Her guilt slowly faded. She bowed again. "Please rest assured, Oyakata-sama. Shinobu and I will do everything in our power to complete the medicine as soon as possible. We'll also ensure complete secrecy. No information will leak."
When she left the residence, night had already fallen. Moonlight shimmered softly across the path.
Tamayo looked toward the Butterfly Mansion, where light still glowed from the lab windows. She knew Shinobu was still inside, working tirelessly.
She quickened her pace, her heart filled with anticipation. Perhaps soon they would succeed—perhaps soon Giyu could become human again, return to his comrades, and once more fight beside them to protect humanity and end Muzan's reign of terror.
Inside the laboratory, Shinobu held a test tube, gazing at the faint blue liquid within. A long-lost smile curved her lips.
She gently swirled the tube, the solution shimmering faintly in the light—like the hope within her heart: small, fragile, yet growing brighter with every moment.