Demon Slayer: Dragon Hashira#38/39: Consuming Dragon Blood, The Mugen Train Begins

Deep Forest—Far From Headquarters

"This should work. It's far enough from Headquarters that I won't expose its location," Kaidō noted, surveying the dense forest where the thick canopy blocked most sunlight, creating a perpetual twilight.

'System, how do I activate the Isolation Barrier?'

[Simply Will it into existence, Host.]

'Got it.'

"Activate."

A visible barrier expanded outward, encompassing a ten-meter radius. From the outside, nothing looked different—the barrier was perfectly transparent, rendering its interior invisible.

'Demons inside cannot escape, right?'

[Nothing within the barrier can exit. Even Blood Demon Arts are suppressed and unusable inside.]

'Perfect! Shame it is single-use though.'

He retrieved the Collection Card from his inventory. Mukago's image stared back—still looking rather suggestive. Shaking his head, he tore the card.

Light flashed, and Mukago materialized before him.

"What?? How did I—" She looked around frantically. "You! You did this!"

"Did what?" Kaidō blinked innocently.

"This! This is your fault!" She raised her hands. "Blood Demon Art: Vine Manipulation!"

Nothing happened.

"Impossible..." Her voice trembled. "Blood Demon Art: Vine Manipulation!"

Still nothing.

Despair washed over her, and she collapsed to her knees.

"Pfft~" Kaidō couldn't help but laugh. Her confused, defeated expression combined with the failed dramatic gestures was genuinely adorable.

"..." Tears welled in Mukago's eyes.

"Wait, why are you crying?" His amusement vanished.

"Why?? I just wanted to survive! Is that so wrong??" Her voice cracked.

"Then I will give you a chance to survive. And a chance at redemption."

"What... what do you mean?" She wiped her tears, sniffling.

"I will turn you back into a human. But you will work for the Demon Slayer Corps to atone for what you have done."

"Why should I trust you?"

"Because I am a Hashira of the Demon Slayer Corps."

Mukago fell silent, processing this. Finally, she spoke. "If I agree... will I have food? Shelter?"

Kaidō's expression softened. She really was pitiful. "I will take care of you. You will not suffer—I promise."

"Then... I accept."

He smiled gently and produced the Purification Agent, handing her the vial. "Drink this. It will transform you back into a human. Muzan's blood cells will be completely purged. He will not be able to find you."

'System, after she turns human, Muzan cannot detect her anymore, right?'

[Absolutely correct, Host. She will be entirely undetectable to Muzan Kibutsuji.]

Mukago stared at the deep blue liquid. Without hesitation, she uncorked the vial and drank. Immediately, agony tore through her body—burning and freezing simultaneously. Cold sweat poured down her face. She clutched her chest, collapsing as waves of pain consumed her.

"It is okay. It will pass soon!" Kaidō knelt beside her, pulling her into a gentle embrace. He cradled her head against his shoulder, stroking her hair soothingly. "You are safe. Everything will be alright."

Warmth flooded Mukago's heart. No one had ever shown her such kindness. The pain gradually subsided. Her horns receded. Her eyes shifted from demonic to human. The four thick red lines on her face faded completely.

After roughly thirty minutes, the transformation was complete. Mukago—no, the girl who had been Mukago—was fully human.

"I'm... I'm human again. Thank god... I'm... sob!" Tears flowed freely, but these were tears of relief.

"Do not cry. You are human now—shouldn't you be happy?" Kaidō wiped the tears from her eyes. He took in her appearance: shoulder-length hair, golden eyes, a surprisingly mature figure for her apparent age, and a round, adorable face. He couldn't resist gently pinching her cheek.

"Can I... can I call you big brother?"

"Huh??" The question triggered a memory—his adopted sister from his previous life asking the same thing: "Can I think of you as my big brother?"

"Is that... not okay?" Her disappointment was immediate.

"Of course it is okay! Now I have family in this world too." His smile was genuine.

"Then... big brother, will you give me a new name? I do not want to be called Mukago anymore."

"Sure. Let me think..." He pondered briefly. "I have got it. From now on, you are Retsuki. I am Kaidō. Retsuki, nice to meet you."

"Retsuki... I love it! Thank you, Kaidō-oniichan!" Pure joy radiated from her.

"Come on. I will take you home." He took her hand, helped her stand, then retrieved his greatsword. Mental energy lifted the blade.

Retsuki's eyes widened. "Oniichan, how—"

"Do not worry, it is one of my abilities. Here—climb on my back. Let's go home together!" He crouched down. Retsuki climbed on, wrapping her arms around his neck.

"Okay!" Her face flushed bright red, but the warmth of his back filled her with comfort.

Kaidō immediately regretted this. Retsuki had transformed back but still wore her original red kimono. As she settled onto his back, the fabric rode up, exposing her slender legs, which wrapped around his waist. Her soft arms circled his neck. His hands supported her beneath her thighs. Both their faces turned scarlet, steam practically rising.

"Let's go!" Kaidō broke the awkwardness first. He stood on the floating greatsword with Retsuki on his back and launched into the sky, soaring toward Corps Headquarters.

Kaidō's Estate

Kaidō landed gently in the courtyard, Retsuki still on his back.

She stared at the traditional Japanese architecture. "Oniichan, this is where you live? It is huge!"

"From now on, this is your home too."

"Really? I can actually live here with you?" Her voice trembled.

"You are my little sister. That makes you family," Kaidō smiled warmly, helping her down.

"Thank you, oniichan!"

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Inside, Kaidō gave her a tour, making it clear she belonged—no need to be nervous.

"There are two bedrooms upstairs. One is yours now. My room is next door, so find me anytime if you need anything."

"Okay! Thank you, oniichan!" Retsuki beamed.

"Get some rest in my room for now. I will prepare yours properly. Tomorrow I will introduce you to everyone at the Butterfly Mansion. When I am out on missions, you can spend time with them—but you will need to help out too."

"I will! I promise!"

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That Night

After ensuring Retsuki was asleep, Kaidō slipped out to the courtyard.

[Ding! Congratulations! Hidden Quest Complete!]

[Hidden Quest: Bonds of Family]

[Reward: +5% Interdimensional Energy]

[Current Interdimensional Energy: 55%]

'Another hidden quest? Two in one day—that is unexpected.'

'System, what triggered the "Bonds of Family" quest?'

[You used the Purification Agent to restore Mukago—now Retsuki—to humanity, granting her redemption. By establishing a familial bond, you freed her from suffering.]

'I see. Though this is probably a one-time thing. If similar situations arise, I doubt I can do this again.'

He gazed at the stars. "Cannot believe how much has happened. I have been here long enough that everything feels... normal."

Returning to his room, Kaidō retrieved the vial of [Dragon Blood]. The crimson liquid glowed faintly. Even through the glass, he sensed the overwhelming power within.

"Once I drink this, my physical capabilities will reach their peak," he studied the vial. "As for evolution... it does not matter. As long as I can keep Rengoku alive on the Mugen Train, that is all that counts."

He uncorked the vial and drank. Immense power surged through him, flooding every muscle and bone. The sensation was overwhelming and agonizing. To avoid waking Retsuki, Kaidō clamped his hand over his mouth, biting down hard. Blood seeped between his fingers.

'Damn it... this hurts... hold on... For everyone's sake. To get home. I have to endure!' His vision swam. Consciousness slipped away.

<><><><><><><><>

The Next Morning

Kaidō woke feeling incredibly refreshed, his body lighter, stronger—fundamentally changed. He started to sit up, then paused. Something was weighing him down.

'What the—?' He lifted the blanket. Retsuki was curled up on his chest like a cat, sleeping peacefully.

'Uh... I did not do anything inappropriate, right? Please tell me I did not. Because if I did, that is three years minimum, maximum death penalty...'

"Mm? Oniichan, you are awake?" Retsuki stirred, yawning adorably as she stretched, accentuating her figure.

"Retsuki, why—wait, why are you in my room? And I did not... do anything last night, right?"

"I am sorry, oniichan!" She looked down, embarrassed. "I was scared sleeping alone, so I came here. I am sorry!"

"Silly girl, I am not mad." He ruffled her hair gently. "You do not need to apologize."

"Really?"

"Really! Now come on, let's get up. I will make breakfast!"

"Thank you, oniichan!"

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After Breakfast

Kaidō purchased a light, flowing dress from the System Lifestyle Shop for Retsuki. Then he took her to the Butterfly Mansion. Girls got along better with other girls.

They arrived at the entrance. Kanae spotted them immediately.

"Oh my! Kaidō-kun, who is this?" She noticed the small girl clutching his hand, peeking out shyly.

"This is my little sister, Retsuki. I will explain later."

"Your sister? She is absolutely adorable! Just as cute as you!" Kanae beamed.

"Uh... anyway, Kanae-nee, I need to ask a favor."

"What is it?"

"I would like Retsuki to work here at the Butterfly Mansion."

"No problem at all! Though, Kaidō-kun, how do you plan to thank me?" She gave him a playful smile.

"Anything within my power."

"Then it is settled! Come on, I will introduce you both at the training grounds."

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Butterfly Mansion—Training Grounds

Kaidō held Retsuki's hand. "Do not be nervous. Everyone here is kind. Plus, we are close by—if you ever need me, I am just a short flight away."

"Okay, oniichan. I will do my best!"

At the training grounds, the medical staff and Tanjiro's group were present.

"Sister! Oh, Kaidō-kun?" Shinobu noticed them approaching with the girl. "Who is this?"

"Shinobu, this is Kaidō-kun's little sister! She will be joining us here!"

"WAAAHHH! SO CUTE!!" Zenitsu's eyes locked onto Retsuki. "Kaidō-san! Is this your sister?? Did you bring her to introduce her to me??" He stared intensely.

"Oniichan, he is scary! Sob..." Tears welled in Retsuki's eyes.

"Do not cry. I will protect you." Kaidō pulled her behind him. His greatsword floated into position, hovering threateningly. Protective-brother mode: activated.

"Zenitsu, you made my sister cry." Kaidō's smile didn't reach his eyes. "Let me give you some advice: do not even think about pursuing my little sister. Otherwise... I do not mind taking over your training personally. And trust me—I will take very good care of all three of you." His grin widened, revealing small fangs, looking predatory.

"AHHH! Tanjiro, he really IS a demon! Look—his sword is floating! That is definitely a Blood Demon Art!" Zenitsu dove behind Tanjiro.

"Zenitsu, quiet! That is Kaidō-san's family technique," Tanjiro sighed, then bowed apologetically. "I am sorry, Kaidō-san. Please forgive him."

"White-hair! I challenge you!" Inosuke pointed dramatically.

"Once I beat you, I will be—" The rest died in his throat. Kaidō's greatsword hovered at his neck, the edge kissing his skin.

'When did it—I did not even sense it move!' Cold sweat dripped down Inosuke's face.

"Alright, alright, everyone settle down!" Kanae clapped her hands, breaking the tension. "From today forward, Retsuki is part of the Butterfly Mansion family! She will be working and spending time with all of us!" She turned to Retsuki warmly. "If you ever need anything, feel free to come to me."

"Do not worry, Retsuki," Kaidō stroked her hair. "Kanae-nee and the others are wonderful people. I have to go on missions sometimes, so you would be bored home alone. This way you can make friends."

"Okay! I will listen to you, oniichan."

"Then I will leave her in your care, Kanae-nee. Retsuki, I will pick you up tonight."

"Do not worry, Kaidō-kun!"

"I will wait for you, oniichan!"

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Time Skip

Over the following weeks, Retsuki gradually came out of her shell, transforming into a cheerful, lively young girl thanks to the others at the Butterfly Mansion.

During the day, she helped care for patients; at night, she returned to Kaidō's estate. When he came home from missions, they would chat.

Sometimes he played the flute for her or taught her origami.

Simple, precious moments.

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One Month Later—Ubuyashiki Estate

Time had flown. The Mugen Train arc was about to begin. Kaidō stood before Kagaya.

"Child, what troubles you?"

"Master, I request permission to accompany the Flame Hashira to investigate the Mugen Train incident."

"Permission granted."

"You are not going to ask why?" Kaidō blinked.

"Because we trust you. Everything you have done for the Corps has been extraordinary," Kagaya smiled gently but firmly.

"Thank you for your faith in me, Master."

"Kyojuro departed three days ago. Tanjiro and his companions left yesterday. Your Kasugai Crow will guide you."

"Understood."

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On the Road

Leaving the estate, Kaidō encountered Genya Shinazugawa.

"Dragon Hashira-sama!"

"Yo, Genya! How is Water Breathing training coming along?"

"I am getting more proficient every day. I am confident I will master it completely soon."

"Excellent work. Find me once you have perfected it." Kaidō reached into his inventory. "Also, take this."

He handed over the Explosive Gun—a reward he'd never used.

Genya stared in awe. Jet black body with crimson light flowing across it, dragon engravings decorating the barrel. Beautiful and deadly. He loved it instantly; it radiated far more power than his old firearm.

"This is the Explosive Gun. Highly effective against demons. You will discover its abilities through use. I am giving it to you—it will serve you better than me."

"Thank you, Dragon Hashira-sama! I will use it to destroy demons!" Genya's determination burned bright.

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Mugen Train Station—Three Days Later

"Finally... made it... I am exhausted."

The journey took three days instead of one due to a demon attack. The demon possessed a concealing Blood Demon Art. Without Hyper-Awareness, finding it would have been a nightmare. Once located, he dragged it into the woods, beat it senseless, then decapitated it before continuing.

WHOOOOO!

The Mugen Train's whistle blew; it was about to depart.

"Wait! I am not on yet!" He sprinted and leaped aboard just as it began moving.

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Inside the Train

Moving through carriages, Kaidō searched for Kyojuro, then heard the unmistakable, booming voice ahead.

"UMAI! UMAI! (Delicious)"

"Perfect timing!" Kaidō grinned. 'I get to witness Rengoku's legendary bento scene!' He headed toward the sound.

"Yo! Rengoku!"

"Hm?" Kyojuro looked up, eyes lighting up. "Young Kaidō! You have arrived! Come, sit! This beef hotpot bento is delicious!"

"Do not mind if I do—I have been traveling for days and am starving!" Kaidō dropped into the seat across and grabbed a bento.

"Kaidō-san! You are here too!" Tanjiro's face brightened.

"Yeah. How have you been, Tanjiro? Is Nezuko doing well?"

"Yes, Kaidō-san! Everything is great! Nezuko is doing wonderfully!" Tanjiro's gratitude was obvious. "Thank you for protecting her back then."

"Do not mention it. Your family took me in. That makes Nezuko my little sister too. What kind of big brother would I be if I did not protect her?"