In the lowest basement of Sakuragi Hospital, metal canisters were lined up, each about a meter tall. Their metal shells blocked any prying eyes, making it impossible to see the specific objects placed inside.
However, through several transparent pipes connected to the canisters, one could clearly see a red liquid being continuously injected, looking exactly like blood.
In fact, it was blood, but not human blood; it was pig blood. Its primary purpose was to provide nutrients to the demons sealed and slumbering inside the canisters, preventing them from being truly squeezed dry by Kael.
Kael was very interested in the potion that created creatures like demons, but when he tried to reverse-engineer the potion's formula from demon blood, he realized it was simply impossible.
The birth of Kibutsuji Muzan was an unrepeatable result.
Presumably, the doctor back then was just treating a dead horse as if it were alive; even he hadn't anticipated such an effect.
Kael had good reason to suspect that the birth of demons, besides the effect of the potion, also involved factors inherent to Kibutsuji Muzan himself.
Reverse-engineering the potion's formula was a dead end, but a ready-made potion was right in front of Kael.
That was the blood of the Demon King.
However, it was mixed with too much of Kibutsuji Muzan's pollution. If it wasn't cleaned out thoroughly, a reckless injection would result in becoming a demon with a warped mind.
Yet, removing the pollution was only the first step.
To restore the Demon Kings blood to the form Kael wanted, more research and experimental materials were needed.
This was the origin of the canisters before him.
Ordinary demons contained a concentration of Demon King blood that was too low. Even if Kael recycled it, he couldn't squeeze much out in the end, and it was now difficult to meet his experimental needs.
If he could capture Kibutsuji Muzan and bleed him, Kael was very confident he could purify and perfect the Perfect Potion he wanted within a few months.
However, reality is harsh. Kibutsuji Muzan was not that easy to catch; one only had to look at the history of the Ubuyashiki family's struggle to know that.
In these several hundred years, only Tanjiro had seen the Demon King and lived.
He was hard enough to find, let alone capture.
If he accidentally gave the guy a heart attack, there was no telling if he'd just dig another hole and bury himself again.
When that time came, he would truly be at a loss.
Kael could only set his sights on the Twelve Kizuki first. The concentration of demon blood in them should be quite considerable. If he could catch one, it would definitely lead to a breakthrough in his research progress.
As for how to proceed, he just needed to keep a close eye on Tanjiro.
A member of the Kizuki would eventually deliver themselves to his door.
Thinking happily to himself, Kael reached out and turned on the basement lights, and the previously dark environment instantly became bright.
Seen this way, the atmosphere was much more cheerful.
Otherwise, if an outsider saw this, they might think he was some kind of villain.
The Arrow Demon and Temari Demon caught two days ago hadn't been processed yet; they were lying on operating tables, having lost consciousness under the influence of various drugs.
Kael skillfully drew their blood and placed it into a self-made centrifuge for the most basic purification work.
His research over these years hadn't been without success.
Although he was still a significant distance from the final Perfect Potion, a "Panacea" that could cure everything already had a finished product.
It was the result of merging knowledge systems from multiple worlds, including Alchemy, biology, and pharmacology.
"Not bad, as expected of fresh stock, the purity is considerable."
Kael picked up the purified demon blood to observe it and nodded with satisfaction.
He divided it into several vials. One part was for research, and the other part was made into the Panacea to nourish Ubuyashiki Kagaya's body.
After being fished out of the water the other day, Ubuyashiki Kagaya's cold hadn't gotten better. He had a runny nose all day and didn't dare go out to see people, saying he feared ruining the Master's image.
It was precisely by relying on these "Panaceas" that Kael was able to maintain Ubuyashiki Kagaya's physical condition.
Besides the Panacea, Kael had also developed other types of potions using demon blood as the primary material.
There was one specifically for external injuries, and the effect was very good.
Even if a limb was severed, as long as it was still viable, it could be reattached directly by injecting the potion externally.
This greatly reduced the casualties of the Demon Slayer Corps.
After all, humans are not demons and are very fragile. When fighting them, even a slight organ injury, even if they survived in the end, would greatly damage their future combat power.
This was also the reason Kael made these canisters and continuously drew their blood.
Consider it an atonement for these demons; that way, if they go to hell one day, they might stay for a shorter time and reincarnate sooner.
Thinking about it this way, he really was performing a great act of merit.
Kael had never hidden his research on demon blood.
At first, it naturally drew some criticism from the Hashira. The mere act of bottling up demons was not something everyone could accept.
It wasn't even about whether it was humane or not.
They had already become man-eating demons; there was no humanity to speak of.
The Hashira were just worried about potential risks. If a demon accidentally escaped and caused new casualties, wouldn't that be Kael's fault?
However, once he produced concrete results, these voices of opposition grew weaker day by day.
Compared to the results of these potions, a small amount of risk was not unbearable.
After a series of skillful operations, Kael packaged the finished potion and handed it to Xiaobai, who would bring it to Ubuyashiki Kagaya.
Then he packaged the Temari Demon and the Arrow Demon into two new canisters.
However, the day's work was not yet over.
In the flower bed outside another hospital building, Kael still had to tend to the wisterias he was growing hydroponically.
These wisterias were all the latest varieties he had improved many times.
The flower buds were as plump as grapes, hanging in clusters.
When they bloomed, the rich fragrance could permeate the entire hospital.
If an ordinary demon even took a sniff, they would be poisoned, and their self-healing ability would be severely weakened.
Besides wisteria, Kael also planted other plants in this flower bed, including the blue spider lily transplanted from the mountain near Tanjiro's hometown.
This thing was indeed difficult to cultivate, with extremely high environmental requirements.
Even with Kael's Botany level, it took nearly a year to make it bloom successfully.
The importance of this blue spider lily goes without saying; it was the key to the "Humanization Potion" and the "Perfect Potion."
Although Kael hadn't extracted any magical elements from it.
This situation was like the Scarlet Crimson Iron Sand and Scarlet Crimson Ore used to forge nichirin blades; Kael hadn't found specific solar energy in them either. It was clear that the uniqueness of both was a result of the conceptual level.
The first batch of blooming blue spider lilies wasn't many. After Kael extracted and preserved their juice, the remainder was only enough to stir-fry a dish, not even enough for Kanroji Mitsuri to eat in a few bites.
To be fair, the taste was indeed good, and the texture was great.
Shinobu Kocho had only recently planted the seeds for the second batch, and it would be some time before they sprouted.
When Kael arrived at the flower bed, Shinobu Kocho was already updating the records of the wisteria's growth data. Her mint-colored haori was like the wings of a dancing butterfly, swaying gently in the gaps of the flower bed.
Her focused and serious face exuded a sense of tranquil beauty.
Kael silently came up behind Shinobu Kocho. The girl's body had long been infused with the fragrance of wisteria, which smelled more high-end than any perfume.
"Don't disturb my work at a time like this."
Even though Shinobu Kocho said that, her body showed no intention of pushing Kael away.
"You do yours, and I'll do mine. We won't interfere with each other."
Seeing Kael's actions getting more and more excessive, Shinobu Kocho couldn't take it anymore. She picked up her pen and poked the back of his hand as a warning.
Seeing that he had almost reached the limit, Kael finally behaved himself.
"It's time to water this flower bed; the soil is a bit dry."
Shinobu Kocho ordered Kael around; in this flower bed, she held the highest authority.
"Little Shinobu is really cute when she's seriously giving orders."
Kael remarked with a sigh.
"..."
Shinobu Kocho gave Kael a light kick.
"Alright, alright."
Kael raised his hands in surrender and started the physical labor.
By the time the two finished tending to the flower bed, it was already late.
Usually, at noon, Kael would stay at the hospital to eat with Shinobu Kocho, but in the evening, he definitely had to go back to be with Kanroji Mitsuri.
As for a late-night snack, that would definitely be with Lady Tamayo.
The schedule was arranged clearly and very reasonably.
Naturally, the house didn't lack chefs and servants, but Kanroji Mitsuri had a significant obsession with cooking for Kael.
Her cooking was indeed good, but considering her own appetite, one could probably guess that this was certainly no simple or easy task.
However, with Kanroji Mitsuri's extraordinary constitution, she didn't need anyone to lend a hand, and she herself wasn't happy about anyone encroaching on her kitchen territory.
"Mitsuri, I brought a guest back. Remember to add an extra pair of chopsticks for dinner."
As soon as Kael entered the door, he shouted toward the kitchen.
"S-Sorry to intrude!"
Tanjiro, with a stiff expression, shouted loudly.
"Is there a guest?"
Mitsuri poked her head out and saw Tanjiro, blinking her eyes.
She had thought the guest would be Ubuyashiki Kagaya or Rengoku Kyojuro.
If it were the former, there was no need to worry; just a few scraps from Kael's mouth would be enough to stuff him. If it were the latter, it would take a lot more effort; this second brother's appetite was no joke.
"Mrs. Sakuragi!"
Tanjiro tensed his body and shouted loudly.
He felt inexplicably guilty.
He unconsciously thought of Shinobu Kocho and Lady Tamayo.
Even someone like Tanjiro, who had never had any romantic experience, could sense that the relationship between Kael and those two was quite unusual.
But Big Brother is clearly already married.
Tanjiro's morals and worldview suffered a significant impact.
"You look quite young; you can just call me sister-in-law."
Seeing Tanjiro like this, Kanroji Mitsuri had a good impression of him, mainly because he reminded her of her own younger brothers, so she spoke kindly.
"Yes!"
Tanjiro sniffled, thinking of his family.
"How is Tanjiro's appetite?"
Kanroji Mitsuri asked.
"Just go with half of your portion."
Kael spoke from the side.
"That's quite good, no wonder you've grown so strong!"
Kanroji Mitsuri had an idea now and, holding her spatula, returned to her battlefield full of energy.
Tanjiro opened his mouth, momentarily speechless.
Half of sister-in-law's portion?
Would that be enough to get full?
Having just arrived, Tanjiro didn't yet understand what kind of energy was contained in Kanroji Mitsuri's seemingly slender frame.
"Sister-in-law, let me help you!"
Tanjiro rolled up his sleeves, prepared to be useful. Just waiting to eat wasn't in his character.
However, Kael grabbed him and lifted him up, his eyes carrying a piercing scrutiny as he spoke.
"Kid! You want to be alone with my wife?!"
"N-No, Big Brother, I..."
Tanjiro's body trembled with fear.
Big Brother is so scary!!!
"Just kidding. Mitsuri doesn't like people disrupting her cooking rhythm; she likes to enjoy the fun in it alone."
Kael's expression changed, and he smiled as he rubbed Tanjiro's hair and led him into the house.
"Whew~."
Tanjiro secretly breathed a sigh of relief, raised his hand to wipe the cold sweat from his forehead, and put down the box on his back.
"Nezuko~ It's dark, time to wake up."
Kael tapped on the box and spoke in a gentle tone.
Beside him, Tanjiro felt a bit uncomfortable.
Big Brother had never spoken to him so gently.
Hearing the voice, Nezuko poked her head out, looking around the unfamiliar environment with her blank little eyes.
"Cute Cute."
Kael lifted Nezuko up.
Nezuko, having shrunk her body, was just like a small child.
Although a brother should be happy that Nezuko was so well-liked, Tanjiro felt a bit envious at this moment.
Clearly, Big Brother's charm was much higher than Sister-in-law's.
"Eh?! Where did this little sister come from?"
Kanroji Mitsuri looked at Nezuko, a bit dazed. Had she not noticed such a cute little girl just now?
"This is Tanjiro's sister, Nezuko. She's currently sick and can't stand the light, nor can she eat normally."
Kael explained.
There was no need to tell Mitsuri about the existence of demons, to avoid unnecessarily causing her distress and worry.
"Waaah~ From now on, I'm your big sister!"
Kanroji Mitsuri pulled Nezuko into her arms, her voice carrying a sob.
"There, there. I'm researching this illness; Nezuko's life won't be in danger."
Kael pulled Kanroji Mitsuri into his arms and wiped her tear-stained face clean.
"Mhm, mhm."
Kanroji Mitsuri snuggled into Kael's chest, having absolute confidence in him. Since it was something he promised, it could definitely be done.
Looking at this family of three, Tanjiro felt like he had become extra.
Wait, something's not right.
Aren't I Nezuko's brother?
Before long, the servants brought out the dinner Kanroji Mitsuri had prepared, one by one.
The three-meter-long table was fully covered, and the aroma was overflowing.
Tanjiro looked at the bucket of rice in front of him, then at his sister-in-law's beside him; he felt that Big Brother might have slightly overestimated him.