Chapter 11: Dosage Alone

After Shinobu Kochou demanded to meet Miss Tamayo, Kiyoshi led her and Miss Kanae into the house. Mister Yushiro seemed shocked to see Miss Kanae here once more, but not as shocked as Shinobu was to meet a demon who didn't reek of hostility and death.

For her part, Miss Kanae seemed content to stay in the main hall and converse with Master Wolf, though how much conversation she would get from him was anybody's guess.

Despite her surprise, however, she managed to regain her composure with a speed that bordered on unnatural. He couldn't help but wonder what drove her to go so far, to be so serious and determined.

Not that Kiyoshi had any room to talk about unnatural maturity; he was starting to think that having to grow up far too fast and far too soon was part and parcel of being a Demon Slayer. From what little Master Wolf had said about the topic, this seemed to be a trait shared with shinobi.

Regardless, when they reached Miss Tamayo's room, some small part of Shinobu's facade seemed to recede. Once introductions had been made, Miss Tamayo encouraged Shinobu to stay and be a part of her lesson with Kiyoshi. Shinobu readily agreed; lessons passed down through hundreds of years from some of the brightest medical minds in the history of Japan—no, in the history of the whole world, were a priceless boon.

And so it came as no surprise that Shinobu became a regular visitor to their home, coming by at first solely for medical studies and then also for advice regarding developing her Breath style, sometimes staying for weeks at a time.

It was plain to see that it frustrated her, but even with the reinforcement provided by Total Concentration Breathing, Shinobu was simply too small, her arms too thin and frail, for her to effectively apply the Breath of Flowers.

Of course, this was where her medical expertise came in. While Shinobu would likely never be strong enough to decapitate even weaker demons with anything approaching real ease, her mind was razor keen and she had an uncanny talent for concocting medical tinctures and poultices. With the assistance and experience Miss Tamayo brought to the table (both in medicine and in swordplay), the two quickly began to develop a way for Shinobu to fight demons effectively: namely, with special poisons carefully made from wisteria, a deadly plant whose lethality was not constrained to humans alone.

Unsurprisingly, Shinobu's talent with medicine also extended to the creation of toxins. As Miss Tamayo was so very fond of quoting, "All things are poison, and nothing is without poison. The dosage alone makes it so a thing is not a poison."

When asked who she was quoting, Miss Tamayo merely smiled demurely and shook her head, much to both Kiyoshi and Shinobu's collective consternation.

Though Miss Tamayo swiftly formed a bond with Shinobu, she certainly did not neglect Kiyoshi in favor of her new apprentice. In fact, the opposite was true. The lessons in swordplay Shinobu received were not lost on Kiyoshi; it was in the nature of a shinobi to appropriate skills and knowledge from whatever they encountered. As for the lessons on the effectiveness of wisteria poisons, well…

Master Wolf and he had begun the process of designing a new prosthetic attachment, as well as drawing up plans to alter another. Well, it would be more accurate to say that Master Wolf was making the designs, while simultaneously showing him how to make sure they were suitable for, as well as compatible with, the Shinobi Fang.

Working on the Fang was an odd experience, Kiyoshi quickly realized. It was as much a test of physical engineering as it was of metaphysical, requiring him to not only ensure the materials he used were shaped, carved, or otherwise altered in such a way that they suited the Fang, but also that they suited his Chi. It made sense, though. The Shinobi Fang was far from just a false arm; it was every bit a marvel of alchemic sorcery as it was a wonder of medical technology.

Of course, as incredible as the device was, it still took time and effort to make certain that new attachments were suitable, so it would be a while yet before the wisteria oil smoke bombs and the toxin capsule for the Sabimaru were ready for more than preliminary testing.

-x-x-x-x-x-

Shinobu Kochou wasn't quite sure what to make of Kiyoshi Ametsuchi at first, perhaps in part because of how much he reminded her of herself. He certainly carried himself more maturely and seriously than others their age, but yet…

Whenever he would fight or train, there was always a sharpness in his gaze and a weight to his presence. It wasn't precisely uncomfortable, but between his missing arm and the steel in his eyes, Shinobu was certain that his seriousness originated from somewhere different than hers did.

She pushed herself beyond her limits for the sake of someone she wanted to keep alive, but Kiyoshi…

Shinobu suspected that he surpassed himself time and time again for the sake of someone who no longer walked among the living. If that was the case, she couldn't say she knew what he was going through, but she imagined it was agonizing.

As such, she treated Kiyoshi with courtesy and friendliness, and the shinobi-in-training reciprocated, albeit in his own subdued way.

Of course, their cordiality utterly vanished the moment they both set foot on the sparring field. Kiyoshi's teachers thought it would be beneficial for two prospective Demon Slayers who were each developing their own Breath styles to face off against one another with regularity, especially since their respective skill levels were far closer to one another than they were to Doctor Tamayo, let alone the enigma that was Wolf.

Shinobu could see the logic in the idea, and as Kiyoshi seemed to have no objections to the idea, the two made a habit of sharpening their techniques on the edges of each other's blades. Of course, since Shinobu fought with highly deadly toxins and an extremely unorthodox blade, a system was implemented to simulate her having poisoned Kiyoshi. Rather than coating her blade in poison, she coated it in paint, and if she inflicted a sufficient number of marks on Kiyoshi, she was the winner. Of course, the number of marks varied depending on where she struck, and keeping up with Kiyoshi was no mean feat either.

His Breath of Silence might've only had one technique as compared to the two she'd worked out for her Breath of the Insect, but when combined with the bag of tricks Kiyoshi utilized in their spars, that made less difference than one might think. Add to that the fact that, as a derivative of the Breath of Thunder, the Breath of Silence emphasized speed over power, and fighting Kiyoshi was like trying to catch an oil-covered eel with only your teeth.

In other words, it was perfect for honing her fledgling Breath, and judging by Kiyoshi's own progress as time passed and their spars became more and more commonplace, she'd bet that he felt the same way as she did.

By the time Fall arrived, Shinobu had begun developing a third technique, had learned more about poisons and medicines in three months than she'd learned in the past four years, and had gained…

Well, she wasn't quite sure. They'd spent enough time together that "acquaintance" felt far too impersonal by half, but there always seemed to be this...this wall that surrounded Kiyoshi. It wasn't something he was consciously doing, she could tell that much, but it nonetheless prevented her from connecting with him enough that she could call him "friend" and feel as though she was being honest with herself.

…

Whatever the intricacies and complexities of their relationship, Shinobu Kochou could say one thing for certain:

She was glad to have met Kiyoshi Ametsuchi.

-x-x-x-x-x-

After three long months of training, learning, and crafting, Kiyoshi and Master Wolf had finally put the finishing touches on the wisteria attachments for the Shinobi Fang. While the blue rust of the Sabimaru was deadly to humans (and apparently to "Ookami", though Master Wolf never explained just what that meant), it was an annoyance at best to demons...alone, that is.

When the blade was fitted with a cartridge that allowed highly concentrated wisteria oils to coat the blade and mix with that blue rust, it would be a different story—in theory, at least. Miss Tamayo had analyzed both the wisteria mixture and the Sabimaru's blue rust extensively, and had determined that the two substances seemed to react well to one another, but there was no way to know for certain until he used it on a demon.

As for the smoke bombs, getting the ratios of wisteria oil to reactant correct had been troublesome, but the end results were spectacular and deadly in equal measure. Of course, his ability to practice with these weapons was limited to training with Master Wolf, as they would be especially deadly to Miss Tamayo and Mister Yushiro, and Kaigaku's hardiness was no match for Master Wolf's.

Speaking of the ornery Breath of Thunder user, Kiyoshi had, through a combination of stubborn persistence and poking at the other boy's ego, convinced him to spar with him again – multiple times, in fact.

On the surface, Kaigaku seemed to be as gruff and unfriendly as ever...but as their spars continued, Kiyoshi began to understand him better. Simply being exposed to his life energy, clashing with it over and over, made him better able to understand it.

Kaigaku's disdain towards Zenitsu's temperament, his irritation at 'sharing' a teacher, his frustration at being incapable of the first and most basic of the Breath of Thunder's forms—over time, it was all laid bare to Kiyoshi through a dance of steel and souls as easily as if Kaigaku had confided it to him verbally.

Kiyoshi still did not approve of how Kaigaku regarded Zenitsu (even if the elder pupil had seemed to ease up somewhat now that the younger was making tangible progress), but with understanding came empathy, and with empathy came greater understanding still.

Kiyoshi could not pretend to have experienced the things that had caused Kaigaku to close himself off so, but buried beneath all that spite, frustration, and arrogance, Kiyoshi was beginning to see the spark of good that Mister Kuwajima had no doubt seen when he'd take Kaigaku under his wing. It was dim and tinged with old, repressed guilt, but it was there nonetheless.

And that fact gave Kiyoshi a measure of hope. Hope that someone—it didn't have to be him—could get through to Kaigaku, could peel away those layers bit by bit to uncover the spark of kindness lying dormant beneath.

Though neither Kiyoshi nor Kaigaku could really call one another "friend", there was something approaching a bond being forged between them, forged of long hours of clashing blades and crackling souls. A bond of grudging, mutual respect for one another's abilities and determination, and for now, that would suffice.

-x-x-x-x-x-

In a room hidden from even the most keen of observers, a man cursed. He cursed the weakness of the tools that he had brought into this world, had so graciously trained and honed into weapons only for them to be broken, one after another. He cursed the audacity of his last tool, that had believed itself to have its own will and thus abandoned him, taking with him three lesser tools as well.

But most of all, he cursed that man, that wretched relic of the past that had nonetheless bested him with relative ease, had forced him to use one of his most precious tools of all.

His fists struck the black walls until his knuckles bled, one red blood and one green ichor, rage and betrayal filling him like the poison that had replaced the blood in one of his arms.

After a time, he ceased his pointless assault on the wall and slumped over, chest heaving. He was not motionless for long, however. Impossibly, he heard the sound of footsteps behind him. Someone else had entered his sanctum, his safehouse?

Inconceivable.

Heads would roll once he found out who betrayed-

As he turned to face the intruder, he froze in place as he was pinned in place by a gaze.

No, by the gaze. The crimson glare of Japan's apex predator, slit-pupiled and shot through with so many veins the irises resembled cracked glass more than eyes.

Muzan Kibutsuji bared his teeth at Hanzou Uzui, and extended his hand.

After a moment, Hanzou knelt before the progenitor demon and bowed his head.

In the next instant, his skull was pierced and his world was fire.