Upon the sickbed lay a young girl, her visage gaunt, enshrouded in a somnolent stupor. Her skin, a pallid hue, spoke not of vitality but of enervated frailty. Cascading tresses of chestnut adorned her form, though disheveled, a semblance of care was apparent in their disarray.

"Is Miss Sayo your daughter?" asked Teru.

"Yes. A fortnight past, a listlessness befell her, and despite numerous doctors, the nature of her ailment eluded comprehension. I implore your diagnosis." Beseeched the poor girl’s father.

Extending his hand, Watanabe Teru took hold of the girl's wrist, delicate as a willow branch, to discern her pulse. He then retrieved his stethoscope.

After a passage of time, he withdrew the instruments, his countenance steeped in contemplation. Beside him, the head of the Minamoto family inquired anxiouly, "Doctor, what is your prognosis regarding her affliction?"

Teru pondered for a moment.

"It appears to be nothing but an ordinary fever and cold, yet..."

At that instant, Minamoto Sayo roused from her slumber. Gazing upon her father, she exerted her strength, endeavoring to rise, a smile gracing her lips.

"Father."

"Sayo, propriety ought to yield at such times. Recline now!"

The man hastened to assist his daughter's descent.

Turning her gaze toward Teru, Sayo’s eyes gleamed. She then turned to her father and made a request, "Father, might you step outside for a second? I want to have a moment alone with this doctor, please!"

It was evident that the head of the Minamoto family cared much of his daughter. Though the request out of sudden seemed odd, he soon exited the room, before which he cast several glances towards Teru. In the room now remained only Minamoto Sayo and Teru.

"How is Brother Gyomei?" she inquired.

It took Teru's some time to recall who Gyomei is. Himejima Gyomei, a Hashira of the Demon Slayer Corps, whom he had encountered a week prior on a train, serving as the guardian to Kagaya.

Yet, how could this unassuming girl be linked to the demon slayers, and how might she have knowledge of their shared affiliation? Was she not here to seek my medical aid?

As if perceiving Teru's perplexity, Sayo spoke:

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"Actually, it was I who dispatched agents to summon you."

A mischievous smile played upon her lips, lending a spark of youthful vitality to her feeble frame.

"What’s going on?" Teru finally seized the opportunity to voice his query.

...

Once upon a time, there was a young girl.

At the tender age of four, she found sanctuary under the benevolent care of a monk, residing alongside other orphans within a modest temple. Onel day, the fragrance of wisteria inexplicably waned, and a demon broke in. Despite the monk’s reminder, the older ones ventured forth to safeguard the blind monk, in search of means to repel the demons. But they were all killed by the demon, only the young girl was left. Determined to shield her, the monk grappled with the demon and managed to kill it after the sun rose.

But when the police arrived, the young girl, overwhelmed by terror, mistakenly implicated the monk as the murderer behind the slain children. The devoted monk was thereby arrested and incarcerated, condemned to meet an impending execution.

Two years hence, the girl, six years of age, was reclaimed by her kin, the Minamoto family.

And this girl was none other than Minamoto Sayo, while the erstwhile monk assumed the mantle of the Hashira, Himejima Gyomei.

...

Sayo recounted this tale with tranquility, punctuating her words with gentle coughs.

"I could offer myriad justifications—my tender age of four, the paralyzing fear that gripped me, and so forth. Yet, I am disinclined to engage in such a defense, for the past is immutable. Had it not been for the subsequent intervention that spared Gyomei Brother, he would have perished long ago. Several years ago, our paths fortuitously crossed, and that was when my clandestine observation of him commenced, leading me to discover you.

I do not seek his absolution. But it is my earnest hope that he learns about the fact that the other children sacrificed themselves driven by an aspiration to safeguard him, not out of fear of danger. Presently, afflicted by this unknown ailment, I might die soon. Would you, then, undertake the task of delivering this letter to Brother Gyomei, doctor?"

Her delicate finger gestured towards a letter perched upon the table, the ink of which appeared recently dried.

"Why not entrust this errand to a retinue of the Minamoto family?"

Sayo's frail form involuntarily shifted forward. Her hand, which had obscured her mouth, revealed a handkerchief now marred by fresh sanguine stains.

"In such case, I’m afraid he might tear the letter apart. It is my aspiration that he learns of the truth of those days, that we forsook him not in fear, but in a solemn vow. And so, Doctor, I ask you to deliver this letter."

With grace, Teru accepted the letter, his demeanor unwavering.

"No probelm. Now, let us address your ailment."

"Do you know about this illness?"

"Well, this illness isn’t common. It is rather akin to... evolution?" Teru's assertion bore an undercurrent of uncertainty. "Your body remains not harmed, yet your symptoms hint at an adaptation, a response to an unknown stimulus. Have you recently partaken of any peculiar food or drinks or whatever?"

Minamoto Sayo shook her head. After a pause, she spoke:

"None. However, I’ve become unaccustomed to sunlight."

Teru blinked, extracting a vial of elixir which he made into a bowl, and then presented to Sayo.

"This elixir offers relief from pain and facilitates bodily restoration. A reserve remains here, to be ingested twice daily, one bowl per instance. At the very least, it should defer the complete onset of the illness. And I will try to find a way to completely cure you."

"Your benevolence is deeply appreciated, Doctor."

The head of the Minamoto household, who had surreptitiously bugged, now entered, offering a profound bow to Teru.