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The Butterfly Mansion was, by design, a place of peace. It was where the wind chime-like sound of Shinobu Kocho’s laughter occasionally drifted, where the gentle patter of rain against the eaves offered soothing background noise, and where the most dramatic sound was usually the soft thwack of Kanao’s training sword meeting the air.
Today, however, the Butterfly Mansion sounded like a crumbling landslide.
“I! AM! NOT! SICK!”
The roar echoed from the infirmary, shaking the medicinal herbs drying in the sunroom.
Aoi Kanzaki, whose nerves were usually tempered steel thanks to months of dealing with the demon slayer trio, felt a familiar vein pulse in her temple. She balanced a tray holding a bowl of plain, lukewarm gruel, a cup of bitter cold medicine, and the thermometer that had sparked the current crisis.
“You are sick, you idiot! Your fever is over 38 degrees, and you’ve sneezed so hard you nearly cracked the ceiling plaster!” Aoi snapped, kicking open the door with her heel.
Hashibira Inosuke, the King of the Mountains, was curled in a nest of blankets, yet still wearing his boar mask. He looked, Aoi noted with a fresh spike of irritation, pathetic. He was shivering violently, but his limbs were thrashing weakly against the restraints of his bedding. His usual loud, aggressive voice was muffled and wet, like a swamp monster gargling through moss.
“IT’S NOT A FEVER! IT’S—IT’S THE BOILING BLOOD OF THE STRONGEST WARRIOR! I’M COOKING THE STUPID MICRO-DEMON YOU KEEP TALKING ABOUT! IT’S BURNING TO DEATH!”
He tried to leap out of bed, but the sudden movement sent a wave of dizziness through him. He stumbled, catching himself with a shaky hand on the mattress, and let out a pathetic, wet sniffle.
Aoi marched over, her expression a mixture of fury and professional concern. “If you call it a demon one more time, I’m strapping you to the bed. It is a common cold. A simple viral infection. It happens when you train half-naked in the mountains during November, you pig.”
Inosuke clawed at his chest, his voice full of melodrama. “A COLD?! THE KING OF THE MOUNTAINS DEFEATED BY… BY SNOT?! IMPOSSIBLE! I DEFY THIS WEAKNESS! IT’S LIES! ALL LIES!”
“It’s physics,” Aoi said dryly, placing the tray on a bedside table with a loud clack. “Now, take this. It will help with the headache and the fever.” She held out the cup of murky brown liquid.
Inosuke recoiled as if she were offering him a bucket of demon blood. “POISON! YOU DARE POISON THE MIGHTY INOSUKE?! IT SMELLS LIKE DIRT AND BETRAYAL!”
“It tastes like medicine, you spoiled brute. It’s what you get for running off and bathing in an icy stream. Drink it, or I will pour it down your throat.”
This was the core of the problem, Aoi thought for the tenth time that morning. Inosuke understood pain, danger, and competition. He did not understand mandatory rest, bland food, or the concept of a temporary, non-fatal illness. Every single instruction Aoi gave him was perceived as a declaration of war against his ego.
In the doorway, Tanjiro Kamado watched the ensuing battle with wide, worried eyes, Zenitsu Agatsuma clinging to his back like a large, screaming sloth.
“Oh, Tanjiro! T-t-this is horrible! Listen to her voice! Aoi is trying to murder him! She’s always secretly wanted to murder all of us! I can hear the fury in her heart!” Zenitsu whimpered, shaking.
Tanjiro frowned, catching Aoi’s scent: sharp frustration mixed with an undercurrent of deep, practical concern. “No, Zenitsu. She sounds… dedicated. She’s trying to help him. Inosuke just doesn’t know how to be a patient.”
Inside, the confrontation escalated. Inosuke had somehow managed to get his hands on the thermometer, holding the delicate glass instrument like a dagger.
“I’LL BREAK THIS TREACHEROUS WEAPON!” Inosuke yelled, trying to bring it down on the table.
“DON'T YOU DARE!” Aoi shrieked, snatching it away with lightning-fast reflexes that rivaled a Hashira’s. She slapped his hand—a gentle smack, but one that carried the weight of a thousand future errands he would have to run for her. “That is a medical tool! You will not damage the equipment! Now, eat your gruel!”
Inosuke sunk back onto the pillow, genuinely defeated by the loss of his battle loot. His nose was leaking under the mask, and he was absolutely miserable.
This is the worst. Inosuke thought, pushing the mask up just enough to stick his tongue out at Aoi. This weakness is sticky and dull. It doesn't even feel good to fight it.
He hated the way his limbs felt heavy and useless. He hated that the world was quiet, muted by the congestion in his head. He hated that he was trapped in this soft, useless bed. But most of all, he hated that the one person whose constant, sharp annoyance made everything else interesting—Aoi—was now reduced to a bossy, worried drone trying to feed him mashed-up rice.
Aoi left the room to retrieve fresh linens, ignoring the twin sounds of Inosuke attempting to bite his pillow and Zenitsu weeping outside the door. She found Kanao in the hall, watering a plant with her usual silent tranquility.
“I hate him,” Aoi hissed, leaning against the wall. “I hate him so much. I hate his arrogance, his volume, his inability to just sit still and take the damn medicine. I hate that he thinks illness is a personal affront.”
Kanao looked up, her blue eyes calm. She didn't speak, but she tilted her head in a familiar gesture that asked: Then why are you the one nursing him?
Aoi sighed, running a tired hand through her hair. “Because if I leave him to Sumi, Kiyo, or Naho, he’ll trick them into bringing him raw boar meat and he’ll be worse by tomorrow. He’s too stubborn to be left alone, and I’m too stubborn to let him defeat my medical expertise with his sheer idiocy.”
The truth, which she would never admit to anyone, especially not the grinning Zenitsu or the earnest Tanjiro, was that she couldn't tolerate the thought of him being genuinely incapacitated. He was always chaos, always movement, and the sight of him lying still, even if it was self-inflicted, made the Butterfly Mansion feel dangerously incomplete.
He’s like a mountain in the middle of the room. When the mountain stops moving, you know something is truly wrong.
She grabbed the linens and returned to the room, ready for Round Three.
Over the next two days, Inosuke tried every single method in his arsenal to escape the tyranny of the sickbed.
He tried to mimic Tanjiro's breathing techniques to incinerate the cold from his lungs. (Result: A violent coughing fit that brought Aoi running with a glass of water. He tried to challenge the mattress to a duel, using his weakened legs to kick the footboard. (Result: A sore shin and a stern lecture from Aoi about bone-setting.) He even tried to be charming, offering Aoi a wobbly, feverish compliment. “Hey, Tummy-Face! You have sharp eyes! Almost as sharp as my blades! Now, fetch me a fish!”
Aoi had looked at him, deadpan, and said, “My name is Aoi. It means 'hollyhock' or 'blue.' And no. You get gruel. The one you have to eat to beat the pipsqueak demon.”
That was her new strategy: Weaponize the Ego. She realized she couldn't use logic, so she used competition.
“The cold is the strongest demon you’ve ever fought, Inosuke. It is hidden, it is microscopic, and it defeated you with a single sneeze. You cannot beat it by fighting me. You can only beat it by recovering faster than any other Hashira or Demon Slayer in history. Every bite of gruel is a blow against the cold. Every minute of rest is a calculated tactical retreat to gain strength.”
Inosuke’s eyes, visible above his raised mask, immediately took on a fierce, competitive light.
A new kind of competition. A duel against a tiny, invisible coward!
“FINE! I will defeat the snot-demon! I will consume the energy of the victor!” he roared, and immediately—and sloppily—shoveled the lukewarm gruel into his mouth.
Aoi watched, stifling a sigh of relief. It was exhausting, but it worked.
It was late in the evening on the third day. Inosuke was lethargic but mostly silent, finally succumbing to the exhaustion of his perpetual war against the germs. Aoi sat by his bedside, not training, not shouting, but simply writing out medicinal notes in a small ledger. Inosuke was watching her. The small lamp cast soft shadows, making the harsh lines of her uniform and her stern features seem softer, less aggressive.
He realized he wasn't annoyed.
When he was fighting, his focus was absolute: the enemy, the blade, the victory. His internal landscape was always loud, a perpetual shout. But now, lying weak and still, the world was too quiet. The congestion in his head muffled everything, turning the usual noise into a flat, dull hum. The only thing that cut through the dullness was Aoi. Not when she yelled, but when she was simply there.
He felt the cool cloth on his forehead, applied with a firmness that spoke of practiced hands, yet without any actual malice. He tracked the sound of her pen scratching on the paper—a focused, precise sound that cut through the silence like a thin, sharp blade. It wasn't annoying noise; it was a specific, necessary signal.
When she stood up to retrieve a fresh glass of water, the change was immediate. The quiet rushed in. The lack of her presence felt like a small, dull ache in his chest—like an injury that hadn't yet bled. It was wrong. It was unbalanced.
He didn't want the noise of Tanjiro's kindness or Zenitsu's whining. He wanted the highly specific, high-frequency sound of Aoi Kanzaki’s presence.
She returned and placed the glass near him, her hands hovering briefly over his forehead to check his temperature without touching. Inosuke seized the opportunity. He lunged out, weakly, and grabbed her wrist.
Aoi froze, her hand mid-air, her blue eyes wide. "What are you doing, you maniac?! Let go! You'll spill the water!"
His grip was pathetic, but his will was not. He pulled her hand down, forcing it to press against his cheek. The skin of her wrist was warm and firm. "Stay," he grunted, his voice a raw, congested whisper.
Aoi struggled briefly, then stopped, realizing he was too weak to pose a real threat. "I'm not leaving, you fool! I'm tending to you! Just... what are you doing?"
Inosuke didn't have the words for the feeling. It wasn't the competitive heat of battle. It wasn't the satisfying crunch of a successful attack. It was the feeling of necessity. She was a piece of him, a required organ that, when removed, caused a deep, disorienting stillness.
"You are my annoyance," Inosuke mumbled, pressing her wrist harder against his cheek. "The strongest one. The best. I need to hear you shout to know if the mountain is safe."
Aoi stared at him. The absurdity of the statement was so perfectly Inosuke that it short-circuited her anger. She felt her face flush, whether from irritation or something else, she didn't know.
"I am not a safety check, you pig!" she whispered fiercely, but she did not pull her hand away.
He still demanded. He didn't ask. He didn't plead. He demanded. And his demands were, inexplicably, always the loudest things in the room.
Inosuke recovered, of course. The King of the Mountains could not be kept down by anything as pathetic as a virus. The day he was officially released, he immediately ripped off his uniform shirt, strapped on his blades, and tried to charge out the main gate, mask and all.
Aoi was waiting for him, arms crossed, two feet from the gate.
"STOP RIGHT THERE!" she screamed, louder and sharper than she had in days.
Inosuke skidded to a halt, a sudden, familiar satisfaction surging through his chest. That's the noise. That's the frequency.
"HAH! What is it, Small-Face?! Do you challenge me now that I'm at my peak strength?" he crowed, flexing his slightly shaky biceps.
Aoi took a step forward, her shadow falling over him. Her expression was furious. "You didn't do a single post-recovery stretch! You forgot to take your last prophylactic medicine! And you still owe me a new thermometer!"
Inosuke felt the warmth of her pure, righteous fury wash over him. It was better than any sunshine. He grinned wildly under his mask. He had won the duel against the cold, and the trophy—this demanding, scolding, necessary girl—was right where he needed her to be.
"SHUT UP, AOI!" he roared back, his voice finally clear and strong. He charged past her, heading toward the training grounds, but not before deliberately snagging the bow out of her hair and running off with it, leaving her yelling in his wake.
He was fine. Everything was loud, demanding, and competitive again. Everything was exactly as it should be. The King of the Mountains was back, and he knew exactly whose unique noise he would chase forever.