Chapter Text
Iguro’s scent radiated disbelief as they returned to the Butterfly Estate.
Nezuko and Kanao sparred like they’d known each other their entire lives. Their movements were two halves of the same dance, and Tanjiro would have to be willfully ignorant to not notice how they reacted to each other. Their scents tangled and lingered, leaving impressions of warmth and contentment behind.
Inosuke was sparring with Hayase, cacking while ducking and weaving through dulled water whips. Watching a clash between someone as carefully graceful as Hayase and as feral as Inosuke was always interesting; the contrast highlighted their different battle styles while also allowing them to adjust to the type of enemy they’d struggle with the most in the field.
Zenitsu, Rui, and Harumu were excitedly talking about the applications of Rui’s fire threads. “I could become an actual demon slayer, even if it wasn’t official. I could use my threads like a nichirin blade!”
“You’re right,” Harumu murmured. “That would be incredibly useful in another situation like the Mugen Train.”
“You should talk to Kanroji-san. She uses a whip, and your threads function pretty similarly at a distance, don’t they? She’d probably be willing to help you train!”
“That’s a great idea!” Rui gasped. “Thank you, Zenitsu. I’ll go write her a letter now!”
Zenitsu laughed as the demon raced into the estate, getting scolded by both Aoi and Akaza for running into the house so quickly. The two of them were discussing patient treatments as they mixed medications and measured out dosages with Senjuro on the front porch, watching everything going on in the courtyard with equally fond grins.
Flowers were being woven into Kyogai’s long hair by Kiyo, Naho, and Sumi, and the demon shielded his attack drum from any accidental hits while they clambered all over him.
“Left please!” Kyogai obligingly hit his left drum, and Naho giggled as she tilted and got a better angle to weave one last camelia into his hair. “There we go! You look really pretty!”
The other two girls nodded enthusiastically. Kyogai smiled at them all, gently ruffling their hair as he murmured, “Thank you, girls. This was very nice of you.”
Shinobu laughed at Iguro’s dumbstruck expression. “I told you that they’ve made a home for themselves here, didn’t I?”
Kyojuro’s booming laughter echoed Shinobu’s, and Akaza’s head jerked up at the sound. His face slowly tinged red. “Kyojuro! Welcome back!”
Senjuro’s scent flared with excitement, but he carefully maintained focus on what he was doing. He had taken to his new role at the Butterfly Estate like a duck to water, and Tanjiro respected the hell out of him for putting so much effort into it regardless of his natural skill.
Aoi snorted at Akaza’s starstruck voice, drawing Tanjiro’s eyes away from Senjuro and giving him a look that screamed ‘Can you believe this guy?’ with a thoroughly amused scent. “Don’t be so rude to Shinobu-sama!” Aoi teased, delighting in the way Akaza flushed further when he realized that the others were back as well.
“Ah, I’m sorry Kocho-san, Tanjiro-sama!”
“How many times do I have to tell you that Shinobu is fine?”
Akaza’s expression became borderline mulish, and Tanjiro had a feeling that they’d be having that argument for a while. ‘He’s the least formal of my demons, at least among the older ones, but he still gets hung up on stuff like this, huh? I wonder if it’s because she’s his mentor in medicine, even if unofficially…’
Tanjiro was drawn out of his thoughts by a sudden movement from Senjuro. The warm, homey scent of a fireplace and cooking flared with excitement as he passed his dosages off to Aoi and ran forward to tackle Kyojuro into a hug.
“Aniue! Did everything go well?”
“Yes! We had very few objections, and even those are likely to change with time!”
“I’m glad.” A pleased flush spread across Senjuro’s face. “Everyone here is really nice. I’d hate for anything to happen to them.” There was a thread of steel hidden in Senjuro’s words, and Tanjiro nearly vibrated with excitement at the protectiveness there.
His instincts were screaming at him. ‘He sees your people as his too. He’s protective and caring and never gives up.’ Even if Tanjiro had never become a demon, he suspected that he would’ve fallen for Senjuro. But as a demon?
His very being was telling him that he’d found his other half.
It was odd. Tanjiro felt no inclination to fight these particular instincts, but he never sensed anything similar from Akaza despite his clear feelings for Kyojuro. ‘Maybe it’s a demon king thing? I guess it doesn’t really matter. Not like there’s anyone I can ask about this either way.’
“Did you want to see those samples now? It will not be difficult to explain them to you, and I need to pen a letter to the Kamados regardless!”
Iguro looked five seconds away from collapsing in shock. “... That sounds like a good idea.”
-
“Tanjiro? Could you come here a moment?”
“What is it Shinobu-san?” Tanjiro peered over her shoulder as she unfolded a letter, holding up several vials of blood in response.
“The samples I asked your family for came in. I still need to draw a baseline sample from you, then I want you to drink one of these yourself and distribute the rest to your demons. Keep a mental note of who gets what sample, please. It’s important for what I’m testing. And bring everyone back with you, while you’re at it. I’ll need samples after as well.”
The names of his family members were written on the sides of each of the six vials, including one from Nezuko. “I will!” Tanjiro agreed with a nod. Shinobu gave him a smile before drawing a sample from his arm and sending him off with a wave.
Since he was already inside, he started searching for Akaza first. The demon was seldom found far from the infirmary, and Tanjiro’s nose told him that today would be no different. The fact that Akaza was currently speaking with Senjuro was just a bonus.
“How do you know so much about medicine anyway?”
“That’s a long story,” Akaza chuckled. “One we don’t really have time to get into now. The short version is that both my father and wife were ill and needed someone to take care of them, and I was that person. Some things you never forget.”
“I see… Well, thank you for helping. Aoi and I appreciate it.”
“Of course. Demons are the ones causing most of these injuries, and I was no different from them before. This is the least I can do.”
“... I think you were.”
“Hm?”
“Aniue wouldn’t be so quick to defend you if you were the same as other demons, and Tanjiro had to see something in you before he turned you or he wouldn’t have done it. I don’t think you give yourself enough credit, Akaza-san.”
Before Akaza could attempt to refute that, Tanjiro poked his head into the room with a wide smile. “He’s right, you know? Most demons wouldn’t have tried so hard to spare an enemy, much less one as strong as Kyojuro-san. And you did that with all of the Hashira you fought over the years. You weren’t a typical demon at all.”
Akaza grumbled under his breath, but the hierarchy of demons made him hesitant to refuse Tanjiro’s words. Senjuro beamed at him. “Hello, Tanjiro! Did you need anything?”
“Shinobu-san sent me with some blood samples! She’s wanting each of my demons to drink one of these and go back to let her do more testing. Well, me too, obviously, but–” Tanjiro’s voice broke into a stammer, flustered by the increasingly warm smile on Senjuro’s face.
“Sure.” Akaza saved him from embarrassing himself further. “But I don’t want a sample from a woman. I haven’t eaten one in centuries of living, and even if the blood is donated… It just feels wrong. I’ll try to work on getting over that.”
“No, you’re fine!” Tanjiro insisted. “That won’t be difficult to accommodate at all!”
“... And you thought you were like other demons, really?” Senjuro muttered, earning a glare from Akaza.
Tanjiro cleared his throat before passing Saburo’s vial over to Akaza. “Drink this. We’ll probably be sent on more intense missions soon, and everyone needs to be in peak condition for it considering my luck.”
“You can say that again,” Akaza snorted before downing the blood. “You’ve run into members of the Twelve Kizuki twice already. I wouldn’t be surprised if it happened again.”
“Don’t say that!” Tanjiro hissed playfully. “You’ll speak it into existence!”
Akaza only laughed at his reaction. “I’ll go give that sample to Kocho-san now. Wonder what she’s looking into? I wasn't really listening when she asked for my blood before…”
Tanjiro was curious about what she was testing too, but he knew that Shinobu wouldn’t keep quiet on it for long. She was always eager to share the results of her experiments with them.
“If you’re looking for the others, Harumu should be in the dojo with Nezuko and Kanao. I’m not sure about the rest though, sorry.”
He gave Senjuro a bright smile. “Thank you!” Tanjiro tapped the side of his nose with a chuckle. “I’ll track the rest down myself, no worries.”
“You have an enhanced sense too?” The question was shocked out of Senjuro; he clearly hadn’t intended to ask it, judging by the sheepish expression on his face.
“I do! I had a great sense of smell long before I became a demon. What do you have?”
“Enhanced taste.”
Tanjiro’s eyes widened. “Oooo, that sounds interesting! We’ll have to talk about how our senses work some time.”
“That sounds fun,” Senjuro agreed with a nod. “I’ll look forward to it!”
“It’s a date then!” Both Tanjiro and Senjuro went bright red at his words. “I mean–! I… I’m gonna go!”
Tanjiro did not run away. He just very urgently needed to distribute these samples.
-
When he entered the dojo, Tanjiro was surprised to see Harumu wielding a practice sword. He moved with a grace that suggested familiarity with the weapon itself, though not with the forms he was being coached in, and a curious hum escaped Tanjiro.
“Tanjiro-sama!” Harumu startled at his presence. “Did you need me for something?”
“Shinobu-san asked me to give everyone one of these vials.” Tanjiro swirled the samples around with a grin. “But it’s nothing urgent! I can leave one with you; she just wants you to give her another blood sample after you drink it.”
“It’s fine, onii-chan! Harumu was due a break anyway.” Kanao nodded in agreement, and it was only then that Tanjiro realized how exhausted Harumu looked.
“Are you alright? We should really get you more blood if you’re straining yourself so much…”
“I’ll be fine, Tanjiro-sama. This is nowhere near as bad as what I’m used to.” Harumu sighed. “I do seem to tire more easily than the others, though. I’m much weaker too. That’s part of why I wanted to try taking up the sword again in the first place.”
Tanjiro’s eyebrows furrowed in concern. “I could give you more of my blood! I was worried about overdoing it and killing you when you were still sick, so I gave you less than the others to start with.”
“I would be honored, Tanjiro-sama, but we’ll set that aside for now. I’d hate to interfere with Shinobu-san’s studies.” Harumu took Hanako’s sample with a smile and a grateful nod, regaining some color in his face after downing it. “Thank you. I’ll be sure to visit her after my next set; I’m feeling a second wind and don’t want to waste it.”
“Sounds like a plan! Enjoy your training; you too, Nezuko, Kanao-san!”
“You know I will!” Nezuko shouted after him with a wild smirk. “It’ll be even better once Giyu-san gets back from his mission!”
“... Take care,” Kanao murmured, barely loud enough for him to hear. It shocked Tanjiro; he’d heard her speak with Nezuko and Shinobu from time to time, but even then it was rare.
“I will, thank you!” he called back with a wide grin. ‘Another friend! I’m so glad that Kanao-san is starting to open up to more people.’
Tanjiro followed the stronger-than-usual smell of lightning out into the courtyard and lit up with excitement at who he saw.
Kaigaku was currently sparring against Hayase with Zenitsu at his side, and all three of them were being thrown off their footing intermittently by Kyogai. ‘Huh. What an interesting battle simulation. Hayase is learning how to deal with the chaos of facing multiple opponents at once, especially ones who know how to work together, while Zenitsu and Kaigaku can safely practice fighting against a demon with unexpected interferences.’
“Kaigaku! It’s been a while.” Electric blue eyes flickered in his direction before returning to the spar in front of him, but the pleased grin tugging at Kaigaku’s lips was obvious.
“So it has! I hear you’ve been keeping busy. Congratulations on the promotion.” Nichirin screeched as it dragged against a sharpened blade of water, and Hayase chuckled at the clear strain the slayer was under. That quickly turned into a pout once Zenitsu jumped back into things.
“What am I, chopped liver?” Zenitsu grumbled.
“I’ll congratulate you when we become the Thunder Hashira together and not a day before.”
“Oh, come on! That’s–”
The bickering boys all stumbled with the beat of Kyogai’s drums. “Focus! This is meant to be realistic, and an enemy will not give you time to argue amongst yourselves.”
Tanjiro walked over to Kyogai as they did just that, refocusing and clashing with one another once more. “They’re certainly spirited. Did something happen?”
“Kaigaku is determined to show Zenitsu that he still has more skill, despite the fact that they’re of the same rank now. Zenitsu is determined to show Kaigaku that he can keep up with him. And Hayase is just enjoying messing with them both in the process, really. Not that I'm any better,” Kyogai chuckled.
Tanjiro snorted. “Sounds about right. Here.” He passed Kie and Shigeru’s samples over to Kyogai. “One of those is for you and the other for Hayase, obviously. Shinobu-san is going to want another blood sample after you drink them too.”
Kyogai nodded and took Kie’s sample for himself. “I’ll pass that along to Hayase. Thank you, Tanjiro-sama.”
“Of course, have fun!”
Tanjiro waved farewell before turning his nose to the sky and tracking down a much farther scent. He was deep into the forest surrounding the Butterfly Estate before he stumbled across Rui and Kanroji practicing their forms, carving indents into the trees around them as they danced around one another.
“Oh, I wish I could make you my tsuguko! You’re so flexible, and you’d take to Love Breathing so well, I can just tell!”
“Thank you,” Rui whispered, sounding shyer than he had around Tanjiro in a long time. “But it’s really nothing all that special. I’m a demon and a small one, at that. Flexibility was needed. Wielding my threads like a whip is easy because I always preferred fighting long-distance when I had to fight at all.”
Kanroji gave Rui a sad look. “You know, there’s a point where humility becomes a lack of self-confidence, right? You’re doing amazing, Rui! You know what?!” The Love Hashira stood taller, stubbornness radiating from her. “Nichirin blade or not, I’m going to teach you how to use my forms! There’s no rule against it, and who knows, you may make some of your own too! Rui, will you become my tsuguko?”
Dumbstruck silence was her answer. A shaky nod caused her to squeal, tackling the young demon in a hug. “Oh, you won’t regret it, I promise! This is going to be so much fun!”
“Congratulations, Rui!” Tanjiro snickered as Rui flushed a bright red, detangling himself from Kanroji and ducking his head.
“Apologies, onii-sama. I–”
“No need to worry! I don’t mind at all! I’m really glad you two get along so well, actually.” Tanjiro bowed to Kanroji, clearly surprising her. “Please take care of him. He means a lot to me, to all of us.”
“I won’t disappoint you!” she declared, and Tanjiro couldn’t doubt her for a second with a scent that determined.
“It’s unusual for you to go far from the Butterfly Estate without a mission, onii-sama.” Rui’s silent ‘Why are you here?’ could not have been any more obvious.
“Got a blood delivery for you. You can take either one, just make sure to go see Shinobu-san afterward.”
“Oh, this must be that thing she mentioned wanting to test… Sure.” Rui delicately took Takeo’s sample with one of his threads, holding it up to his mouth with a small smile. “I’ll have to write our family a letter when I get back. It wouldn’t feel right to not thank them.”
“They’ll love that!” Tanjiro planned to send them a letter himself, albeit he was more focused on asking after everyone and ensuring that there hadn’t been any odd movements from demons near them. “I’ll leave you two to it. Be careful!”
“Tell Shinobu-san hello for me, Tanjiro-san! I’ll probably head back to my estate after today.”
“Are you going to go with her?” he asked Rui.
“Not this time. I’ll train with Kanroji-san in the times between missions, and we both know they’re planning to send us on a big one soon. I don’t want to miss time on the field.”
A wave of relief hit Tanjiro then. He would’ve accepted Rui’s decision either way, but knowing that one of his main fighters wasn’t going anywhere was reassuring. “Then I’ll see you back at the estate.”
Tanjiro headed back, taking Nezuko’s sample and giving his own to Shinobu. He eagerly awaited whatever results she got from this test.
-
“I was right!” A manic grin spread across Shinobu’s face as she burst into the room, contrasting the dark bags beneath her eyes. “I knew there was something going on with you guys’ growth rate!”
“Are you alright, Shinobu-san?”
She waved Tanjiro’s concern off immediately. “I’ll crash after this; it’s fine. But! Your siblings’ blood is special, Tanjiro, at the very least for your demons! It’s the equivalent of a marechi. I’m guessing that came from your father’s side of the family since neither Kie-san nor Saburo-san’s blood is as potent. Kyogai and Akaza grew at a more reasonable rate between samples, but everyone else’s power levels jumped drastically.”
“Really?” Harumu hummed as he tilted his head. “Come to think of it, training has been slightly easier since…”
“Even then, Akaza and Kyogai grew beyond what one would expect from such a small amount of blood. I’ve got a theory for that now. Your family has worshipped Hinokami for generations, yes?”
“He’s always been our patron deity, even back before demon slayers were a thing.”
“Considering your powers and everything else about how you and your demons function, I have reason to believe that you all gain power from the sun itself. You’re not just capable of standing in the sunlight; you grow stronger from doing so.”
“That makes entirely too much sense,” Akaza muttered. “I’ve noticed that I felt more energized after going out during the day, but I thought that was excitement from being in the sun again that would fade with time. If it truly isn’t…”
“Will you help me test this?”
“After you sleep,” Tanjiro insisted, and while she grumbled about it, Shinobu did concede after Nezuko and Kanao shot her pleading looks. The high-five they exchanged behind their backs terrified Tanjiro. ‘My sister is corrupting Kanao-san,’ Tanjiro lamented, silently saying farewell to any chance of peaceful days that remained.
Once Shinobu woke up again, a full 24 hours later, she had the demons stay out in the sun all day and took samples to compare their previous ones to. The evidence was conclusive; they did grow ever-so-slightly stronger simply by being in the sun. It rejuvenated them, made using their Blood Demon Arts easier, and while none of them were willing to test it needlessly, they suspected that their regeneration would be much faster out in the sun as well.
‘Thank you for this blessing, Hinokami. I swear to you that I will continue honoring you in everything that I do.’
The next several days passed in a flurry of training and preparations both for the humans and demons. Everything was amiable, and while it was chaotic, the Butterfly Estate truly felt like one big family. That atmosphere was only interrupted when another Hashira’s scent entered the mix.
An enraged snarl echoed Aoi’s scream.
“Get your hands off her!”
“Hm? I don’t recognize you. Must be Waxing Moon Three, then. What makes you think I take orders from a demon?”
“What makes you think I respect them from men who kidnap children?!”
Tanjiro arrived just in time to see Nezuko give Uzui a truly cutting glare, preventing him and Akaza from coming to blows. “Hashira or otherwise, you need Shinobu-san’s permission to take one of her girls on a mission. Your orders mean nothing without that.”
“That only applies to tsugukos,” Uzui scoffed, turning to take both Aoi and Naho with him.
Kanao stopped him in his tracks, digging her feet into the dirt and refusing to budge. “Naho… isn’t even a slayer.”
Rui used that moment of distraction to grab both Aoi and Naho, cutting Uzui just enough to force him to release them. Naho devolved into tears and hid behind him, and while Aoi remained more composed, it was clear that she was shaken too. Akaza stood between her and the Hashira, daring him to take another step closer.
“Come on!” Uzui groaned. “Could you be any less flashy? I need a female member of the corps for my mission!”
Nezuko cocked an eyebrow at him. “I am right here and always itching for a fight. Why was your first thought to resort to abduction? Of a noncombatant member, no less? How many of our members do you think could have died because you stole Shinobu-san’s primary medic from them?”
“Honestly, does he have rocks where his brain should be?” Kaigaku sneered, having been drawn in by the chaos.
“I’m a Hashira, you brats! I have superiority over you!”
“Not over me.” Tanjiro stepped into the courtyard with disapproval radiating from him. “If you need aid, then ask for it. You’ll be able to have an optimized team that way; just grabbing the first person you see is setting you both up for failure. So tell me, what’s the mission that has you acting more rudely than half the demons I’ve ever met?”
Nezuko almost choked on her laughter. “Damn, onii-chan, tell it how it is.”
Tanjiro wanted to stay furious with Uzui, and to a certain degree he still was, but he had a feeling that the Hashira wasn’t thinking clearly. He reeked of desperation.
“My wives were investigating the entertainment district in Yoshiwara for likely demon activity. They’ve missed their last two check-ins. They’re former kunoichi and more than capable of handling themselves in a fight, so hearing nothing from them is a cause for concern! We cannot keep wasting time like this!”
“Why didn’t you say so from the beginning?” Nezuko sighed. “Onii-chan, call the others. Looks like we’re going on that big mission earlier than planned.”