In a quiet corner of the Rengoku estate, the air was still and peaceful. Arata sat by the bedside of Rengoku Ruka, though he did not look like the hooded phantom who haunted the woods. Using his biological control—a subtle form of biokinesis—he had shifted his features into those of a humble, wandering doctor.
While Arata had taken up the sword to hunt his father, his heart remained that of Makhihara Ayato. The memories of his past life as a great physician were a calling he could not ignore. He had encountered Ruka by chance months ago, finding her withered by a wasting illness. If it hadn't been for his interference and his advanced knowledge of the human body, Ruka would have been dead by now.
He checked her pulse, feeling the steady rhythm of a heart that had regained its strength. He was pleased to see the dark lethargy of the disease had finally retreated.
Ruka smiled with genuine warmth.
Arata felt a flicker of quiet satisfaction. He reached into his medical bag and produced a small vial containing his final preparation.
He stood and bowed, slipping out of the room and vanishing into the evening shadows just as the front gates creaked open.
Rengoku Shinjuro walked into the house, the weight of his fruitless search for the mysterious swordsman visible in his tired eyes. But the moment he entered the room and saw his wife sitting up, looking healthy and vibrant, his face transformed with pure joy.
Shinjuro breathed, kneeling by her side.
Shinjuro laughed with relief, feeling a deep debt to the physician. He had no idea that the doctor who had saved his wife was the very same swordsman he had been desperately trying to track across the mountains. The healer and the warrior were the same man, and the secret remained safe in the heart of the shadow.
******
The night was shattered by the sound of splintering wood and a boy's desperate scream. In a small, dilapidated house, Sanemi Shinazugawa stood protectively in front of his younger brother, Genya. Before them was a creature that had once been their mother—now a pale, snarling beast with eyes devoid of any recognition.
Sanemi gripped a small knife, his knuckles white, ready to do the unthinkable to protect his siblings. But before he could lunge, a blur of motion swept through the room.
The hooded swordsman, Arata, appeared as if from the shadows. With a speed that defied the eye, he bypassed Sanemi and pinned the demon mother to the floor. He didn't draw his black blade to strike. Instead, he reached into his cloak and pulled out a shimmering glass syringe.
Arata didn't hesitate. He injected the serum directly into the demon's neck. Almost instantly, the creature's frantic thrashing ceased. Her breathing slowed, and the wild, predatory light in her eyes faded into a heavy, forced sleep.
Arata looked up at Sanemi, his red eyes glowing beneath his hood.
Genya looked on from the corner, his face pale and confused. He didn't understand who this man was or how he knew about his brother's blood, but he saw the monstrous form of his mother finally at peace.
Sanemi fell to his knees, tears finally breaking through his hardened gaze.
Arata stood, the chain of his flail rattling softly as he stepped toward the door.
Without another word, Arata vanished into the darkness of the night. Sanemi watched the empty doorway, his heart filled with a gratitude he couldn't name, while Genya remained frozen, wondering if the man he had just seen was a savior or a ghost.
******
The silence following the hooded man's departure didn't last long. The sharp scent of ozone and the heavy tread of iron-shod boots approached the Shinazugawa house. A group of Demon Slayers, dispatched by the Corps specifically to track the mysterious swordsman, burst into the room.
They found Sanemi standing guard over his sleeping mother, his knife still in hand, while Genya huddled nearby. The Slayers immediately drew their blades at the sight of the demon on the floor, but Sanemi threw himself in their path.
He quickly recounted everything that had happened—the hooded man, the serum, and the instructions regarding his own Marechi blood.
The Slayers hesitated. Usually, their orders were absolute: kill any demon on sight. However, they had seen the aftermath of the swordsman's work before. They knew the "Ghost" didn't make mistakes. Looking at the peaceful, stabilized state of the woman, they made a choice that defied their usual code.
In the weeks that followed, Sanemi and Genya were brought into the world of the Demon Slayer Corps. Once they understood the true nature of the monsters that walked the night, both brothers made the choice to join the ranks. Sanemi, fueled by a fierce protective instinct, grew particularly close to another Slayer named Kumeno Masachika.
Together, Sanemi and Kumeno took on the responsibility of caring for Sanemi's mother, keeping her hidden and fed as the hooded man had instructed. While the Corps' leadership remained anxious about a rogue warrior using their sacred Breathing Styles, Sanemi didn't care about rules or unauthorized techniques.
Sanemi told Kumeno one evening while they sharpened their blades.
Sanemi's search became a dual mission: to slay the demons that threatened families like his, and to find the man with the glowing red eyes who had shown a monster mercy.