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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.
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