A Solitary Shadow and Silver Light Under the Moon (Kokushibo)

The order was conveyed directly through blood, with Muzan's unique and unquestionable coldness and boredom.

"That rare blood I found is in your hands, Kokushibo. Teach her swordsmanship and keep an eye on her. Don't let her die. She... might be a little special."

special?

Kokushibo, or rather, Tsugakugo Iwakatsu, slowly opened those six blood-red eyes within the shadows of the Infinite Castle, devoid of any emotion. He had long since become numb to Muzan-sama's supposed "specialness." Centuries of pursuit and loss had tempered his heart to a cold, hardened state, like the alien katana he wielded. Ultimately, the so-called "specialness" was nothing more than another stepping stone on the road to the "highest," perhaps useful or not, or... another disappointing, defective product.

When he first saw the little girl, held captive in a designated area, he didn't see anything "special" about her. She huddled in a corner, her long silver hair flowing like moonlight, a stark contrast to the dim surroundings. She was tiny, her eyes filled with bestial alertness and... a hollow, dead silence. He'd seen that look too many times, in the eyes of those about to be devoured or tortured to death.

Muzan-sama seemed to have a strange kind of "leniency" towards her. He didn't treat her haphazardly like other experimental subjects, but he wasn't being merciful either. He just observed her like an interesting petri dish.

He began to follow his orders, teaching her swordsmanship. Initially, it was just the basics of holding a blade and standing. He taught mechanically and coldly, as if completing a task. The girl learned quickly, incredibly quickly. Her body seemed naturally suited to combat: flexible, coordinated, and... a strange affinity for the moon.

He taught her Moon Breathing. Not out of a desire to teach her everything he knew, but simply because it was the only breathing technique he knew, and what he considered the strongest. He wanted to see how far this being, whom Muzan-sama called "special," could go.

The girl, named Shiro Inji, was as silent as a shadow. She never asked questions, simply imitating and practicing over and over. Her progress was astonishing, but Kokushibo remained unfazed. He had seen countless gifted individuals, including his younger brother, Yoriichi Tsugikuni, who had been a nightmare to him throughout his life. Shiro's talent paled in comparison to Yoriichi's radiant, sun-like brilliance.

Changes are happening quietly.

During the countless nights he spent guiding her in swordplay (there was no day or night in Infinite City, but he was accustomed to moving around during his imaginary "night"), he noticed that Bai would occasionally steal glances at him. It wasn't with fear, nor with admiration, but rather with a kind of... exploration. As if she was trying to discern something from his hideous, inhuman face.

Once, she overtrained and fell, her delicate arms scraped against the rough ground, oozing blood. Kokushibo looked on indifferently, ready to see how she would get up on her own. However, Shiro stared at the blood on her arm, stunned for a moment, then raised his head, staring at him with those dark eyes, and asked softly:

"Teacher...does it hurt when you get hurt?"

At that moment, Kokushibo's frozen heart seemed to be pierced by something extremely subtle.

pain?

He had long forgotten what pain felt like. As a ghost, injury meant regeneration, a trivial process. But the word "pain," along with all those fragile human emotions, had long been rejected by him in his obsession with power.

He didn't answer. But from then on, he noticed that there was a faint, incomprehensible tinge in Bai's eyes when he looked at him. It wasn't a reverence for the powerful, but more like... sympathy? Ridiculous.

He had seen Muzan-sama perform "experiments" on her. He'd draw blood, inject various drugs, and observe her reactions. She always gritted her teeth and endured, neither crying nor screaming, only her clenched fists and pale face betraying her pain. Once, after an experiment, she had a high fever and huddled in a cold corner, shivering. Kokushibo, for some unknown reason, paused beside her for a moment, doing nothing, simply standing there.

The next day, she woke up and saw him still standing there (perhaps he just didn't move all night), and she weakly gave him a very small expression that was hardly a smile.

"Thank you...teacher."

What's there to thank him for? He didn't do anything.

But these subtle, illogical interactions, like water dripping through stone, began to leave marks on the icy walls of his heart. He began to unconsciously check on her condition between carrying out Muzan's other orders. He was still strict and merciless in his swordsmanship instruction, but he would stop before she exhausted herself and reduce the intensity of the training when she was weak from the experiments.

He told himself that this was just to fulfill Lord Muzan's order and ensure the survival of this "special" experimental subject.

Until that day.

When Haku was eleven, she stumbled upon Upper Moon Two, Douma, "eating." The bloody and brutal scene was the final straw that broke her spirit. She suffered from a high fever for several days, her consciousness blurred, and she kept muttering, "Don't...eat people..."

Lord Muzan was displeased with this, believing her spirit to be too fragile. One night, Muzan descended upon the dying Bai, his cold eyes sweeping across her.

"It seems that there is still too much 'humanity'. Since you can't bear it, then you will become a ghost completely."

Kokushibo stood by, watching in silence as Muzan injected more blood into Shiro's body. He watched the girl struggle in excruciating pain, her body undergoing a transformation, ultimately transforming into a demon. Throughout the entire process, he was a true spectator.

However, when Bai transformed into a ghost, her dark pupils, though still filled with fear, did not reveal the bloodthirsty madness of other newly born ghosts. She looked at him with a complex expression: fear, bewilderment, and a hint of... unwavering dependence.

Lord Muzan seemed very satisfied with the results of her transformation, especially after discovering that she still maintained a clear will and did not show the fear of sunlight like ordinary ghosts. He transferred Bai to a deeper and more heavily guarded laboratory for more "in-depth" research.

Kokushibo saw her less and less. Occasionally, when he "passed by" outside the laboratory, he would see her imprisoned on an instrument, with various tubes inserted into her body. Her eyes were more empty than before, and only when she saw him would a faint light light up.

For the first time, a tiny, almost unconscious, doubt grew in his heart towards Muzan-sama... a doubt he hadn't even realized yet. Was it really necessary to treat a child (in his long life, eleven was truly just a child) like this? For the sake of a so-called "perfect life"?

Three years. She spent three years in the lab. Kokushibo could feel her strength growing, but her spirit withering away. Until that night, he "happened" to discover a minor oversight in one of the lab's defense nodes, "happened" to be "inspecting" nearby at that exact moment, and "happened" not to immediately repair the flaw or sound the alarm.

He watched the slender, silver figure, relying on the skills learned from him and the strength gained over the years, quietly escape from the cage.

He didn't stop her. In fact, when she disappeared at the end of the dark corridor of Infinite City, a trace of relief, a light he couldn't even interpret, flashed across his six blood-red eyes.

Let her go. Perhaps she could find another path, different from his.

In the years that followed, he continued as the First Upper Moon, carrying out Muzan's orders and slaughtering demon hunters. But he would occasionally lose focus, using the fragmentary visions Muzan-sama occasionally shared about the "defecting experimental subject" to track her down. He knew that she had encountered the water column, learned to breathe water, and joined the Demon Slayer Corps.

Angry? Perhaps a little. He had taught her the Breathing of the Moon with all his heart, and she was actually trying to learn the "inferior" Breathing of Water? But more than that, it was a complex feeling that was hard to describe. He saw her walking in the sunlight, saw her interacting with humans, saw an expression on her face that he had never seen before, called "smile."

That smile, like a thorn, pierced the deepest part of his heart, reminding him of something he had buried with his own hands a long, long time ago.

When Muzan ordered him to bring her back, he was conflicted. He found her, and he met the human swordsman she had risked her life to protect. He severely injured Tomioka Giyu, driven both by the mission and by a paternal rage that he himself refused to acknowledge—what merit had this human taken?

However, when she drew her sword against him without hesitation for that human, even leaving scars on his body, what he felt was not the anger of being betrayed, but a kind of... deep exhaustion and enlightenment.

She had found it. She had found what he had sought for centuries—a bond he was willing to wield his sword for, even to die for.

When he faced her again in Infinite City and learned she was pregnant, his rage was genuine. It was as if the jade he had meticulously carved (though he had never admitted it), believing it to be unique, had been completely tarnished. But when she cried out "Teacher," speaking of the "love" and "home" she had received, when her resolute gaze met the calm yet powerful gaze of Yunyi in his memory...

He understood.

The path he took led only to emptiness and loneliness, while the path she chose, though fragile and short-lived, was filled with warmth and truth.

So he put down his knife and took the bottle of potion, which was likely hopeless. Not to redeem himself, as he had long since given up hope. Perhaps, it was simply to respond to her repeated calls of "Teacher," to the faint yet persistent light of trust in her eyes that never extinguished.

In the final battlefield, he fought against Muzan side by side with her, drank the potion in the sunshine, and turned back into the strange and old self named Tsugukuni Yansheng... All this seemed like the end of a long and absurd dream.

When she pulled her husband and threw herself into his arms, after that moment of stiffness and strangeness, she felt a warmth she had never experienced before, like sunshine melting ice.

He lowered his head, looking at the happy smile of his daughter (he finally accepted this identity in his heart) in his arms, looking at his silent but determined son-in-law, and feeling the true warmth of the sun...

Perhaps this wasn't redemption, but as an ending after so many dark years, it didn't seem so bad.

He raised his hand, which had just regained human consciousness and was still trembling a little, and finally, gently fell on the white silver hair, like moonlight, finally gently touching the warmth of the world that he had only been able to look forward to for a long time.