Kazama Aoi had shrunk. Yes, you read that correctly—shrunk in the literal, physical sense.
It all started like this...
[Flashback]
"Tanjiro, look out!"
The enemy was a lanky, noseless demon. Its Blood Demon Art wasn't a physical strike, but a cloud of purple mist that exuded an eerie, cloying sweetness.
Aoi only had enough time to shove Tanjiro out of the way before the mist engulfed her head-on.
The expected corrosion and agony never came. Instead, she felt a bizarre sensation, as if the very essence of her body was being drained. Her bones, muscles, and even her clothing began to contract in an impossible, rapid fashion.
"Aoi!" Tanjiro's terrified voice seemed to echo from a great distance.
When the mist finally dissipated, Aoi was nowhere to be found. In her place sat a tiny girl, barely the size of a palm, wearing a crumpled, oversized Demon Slayer uniform. She sat on the ground in a daze, her Nichirin Sword hitting the dirt beside her with a disproportionately loud clatter.
"Aoi... Aoi-chan?" Zenitsu rubbed his eyes, unable to process what he was seeing. "Wh-what happened to you?!"
"What's this?!" Inosuke stopped swinging his dual blades and leaned in curiously, poking Aoi's tiny head with a finger. "She's so small! Like a doll!"
"Inosuke! Don't just poke her!" Tanjiro scrambled over and carefully scooped the shrunken Aoi into his palms. He held her with frantic delicacy, his face etched with anxiety. "Kazama-san, are you alright? Do you feel sick anywhere?"
Aoi shook her small, dazed head. She tested her tiny arms and legs; though her body had diminished, her senses and consciousness remained intact.
She looked up at the three boys, who now loomed over her like literal giants. When she spoke, her voice had transformed into a high-pitched, squeaky trill. "I... I'm fine. I've just... shrunk."
The voice was so soft and precious—devoid of her usual sharp, efficient tone—that Zenitsu's heart nearly did a backflip.
"Whoa! Aoi-chan's voice is so cute too!" Zenitsu cried, hearts practically throbbing in his eyes. "It's a tragedy, but she's so adorable!"
"Now is not the time for that!" Tanjiro, holding Aoi close, paced in worried circles. "We have to find a way to turn her back! We need to get to the Butterfly Mansion and let Kocho-san take a look!"
Sitting in the warmth of Tanjiro's palm, Aoi's feelings were... complicated.
[The Butterfly Mansion]
Zenitsu refused to leave her side for even a second. The hearts in his eyes hadn't faded once as he kept muttering things like "so cute" and "I must protect Aoi-chan."
Inosuke, meanwhile, was fascinated by the novelty of it all. He kept trying to poke her cheeks or inspect her tiny feet, only settling down after Tanjiro gave him several stern, warning glares.
"I haven't found a way to restore Aoi-chan yet," Shinobu Kocho said. Her trademark sweet, unshakeable smile was in place, but Tanjiro's keen nose caught a flash of fleeting gravity in her scent.
"However, I will develop an antidote as quickly as possible."
"So, the next question is," Shinobu's gaze drifted over the three boys, "who will look after Aoi until she recovers?"
"Me!"
"Me!"
"Me!"
Zenitsu, Inosuke, and Tanjiro shouted in unison.
"My, my. It seems Aoi-chan is quite popular. But we should really hear what Aoi-chan thinks about this."
Shinobu's suggestion instantly silenced the three boys, who had been on the verge of a shouting match. All eyes—including Kanao's, which usually remained somewhat vacant—landed on the tiny figure in Tanjiro's hand.
Aoi felt a bit conflicted. She cleared her tiny throat. "Ahem. I've decided. I want to stay with Kanao."
Though her voice was squeaky, the words hit like a stone dropped into a still pond, sending ripples through the room.
"Ehhhhh—?!"
Zenitsu looked like a cat that had just had its tail stepped on. He stared at Aoi in disbelief, tears falling like strings of broken pearls. "Wh-why, Aoi-chan?! I would protect you with my life! Kanao... she doesn't even talk!"
Tanjiro looked down at the tiny Aoi in his palm and asked softly, "Aoi, staying with Kanao-san is a wonderful idea... but are you sure? We... we could take care of you, too."
Aoi nodded firmly. Being with another girl was simply more practical for certain things. Besides, Kanao's quiet, steady nature was a welcome reprieve from the "passionate" energy that usually gave Aoi a headache.
Standing silently behind Shinobu, Kanao Tsuyuri felt her body stiffen slightly when her name was mentioned. Those eyes, which usually seemed hollow, focused on Aoi with a rare hint of genuine confusion.
Shinobu's smile deepened. "Oh, it seems Aoi-chan has made her choice. Kanao, are you willing to take on this mission?"
Kanao didn't answer immediately. Her gaze flickered between Aoi and Shinobu. Then, as if remembering something, she gave a silent, solemn nod.
"Then it's settled," Shinobu announced with an air of finality. "Tanjiro, Zenitsu, Inosuke—you are welcome to visit Aoi-chan whenever you like, but please do not disturb her excessively while Kanao is caring for her. This is Aoi-chan's own wish."
"Ugh..." Zenitsu wanted to protest, but under Shinobu's "gentle" yet razor-sharp gaze, he swallowed his words, letting out only a pathetic, whimpering sob.
And so, "custody" of Kazama Aoi was dramatically transferred to Kanao Tsuyuri.
Kanao's room was tidy and silent. Much like the girl herself, it possessed a pristine, tranquil atmosphere. The air was filled with the faint, comforting scent of soap and medicinal herbs—the signature aroma of the Butterfly Mansion.
The room was sparsely decorated: a bed, a low table, and a shelf against the wall holding a few books and simple tools. Dust motes danced in the shafts of sunlight filtering through the shoji doors.
Kanao brought out an unused, exquisite wooden jewelry box. She lined it with her favorite soft handkerchief—one embroidered with a cherry blossom pattern—and then gently lowered Aoi into it.
"Is this... alright?" Kanao knelt beside the box. Her voice was as soft as a summer breeze.
Lying on the plush handkerchief, Aoi found it was a perfect fit. She looked up at Kanao's refined face, which held a trace of lingering bewilderment.
Aoi nodded. "Mhm. It's very comfortable. Thank you, Kanao."
Kanao, seemingly unaccustomed to such direct gratitude, instinctively averted her gaze. A nearly imperceptible blush colored her cheeks.