Chapter Text
At the Rengoku household, a thirteen-year-old boy was practicing with a wooden katana, trying to refine his stances in the quiet atmosphere—but that calm was soon broken by his father.
"Senjuro! Come here and bring me another jug of alcohol!" his father shouted from his room.
Ever since the visit from the young demon slayer with crimson eyes, his father hadn't left his room, calling out every so often for another jug of alcohol.
"I'm coming, Father!" Senjuro went to the kitchen to fetch the jug. "It's almost running out," he murmured.
His father had been an alcoholic since Senjuro was a baby; the habit had worsened after the death of his older brother, Kyojuro. His anger had intensified further when he learned that the Demon Slayer Corps had never recovered his brother's body—according to the slayers, it was as if the body had simply vanished into thin air.
Senjuro was all alone now. His only remaining loved ones were his adoptive brother, Obanai Iguro, and his brother's apprentice, Mitsuri Kanroji—but they couldn't visit him due to their duties as Hashira. His father had become a stranger to him; Senjuro stayed home only to look after the man and keep him from being alone, knowing that leaving would mean disappointing his late brother.
"Senjuro, hurry up! I haven't got all day!" his father yelled.
Shinjuro had very little patience—a dangerous trait for Senjuro, given that his father became aggressive whenever he was in a bad mood, which was practically all the time.
"I'm coming, Father!" he said, grabbing one of the remaining jugs. Young Rengoku hurried toward his father's room, a jug cradled in his arms. He was rushing to deliver the alcohol, hoping that—with any luck—his father would fall into a deep sleep rather than forcing him to train until midnight, finally allowing him to get a full eight hours of rest.
Upon entering the room, Senjuro was startled as an empty alcohol jug was hurled to the floor; it shattered into a thousand shards, some of which skittered toward his feet, slicing his skin.
"Hurry up, you idiot!" his father shouted from the bed.
He was sitting up, though he swayed unsteadily; his words were slurred and unintelligible due to his intoxication. He rose from the bed and snatched the jug right out of Senjuro's hands.
"Can't you even do *that* right?" he sneered, shaking the jug before taking a swig. "Pfft... I can't believe I raised a good-for-nothing."
"Father, please... don't call me that. I’ve tried everything to get my sword to change color, but I just can't manage it. I just need a little more time." Senjuro was fed up with his father's behavior; he no longer felt affection for the man—he harbored a certain hatred instead. "I don't know... maybe I could make progress if you actually helped me." Shinjuro paid no heed to his son's words. Finally weary of being ignored, Senjuro raised his voice. "I just need you to help me—that's all! Or is even *that* too much for you?" Silence filled the room; the anger of both Rengokus was mounting, and the atmosphere grew heavy. Neither could stand the other at that moment. The last thing Senjuro heard before collapsing to the floor was the movement his father made to hurl the jug he was holding at his head; since the drink wasn't finished, the alcohol trickled down his yellow hair, soaking all the way to the red tips.
"YOU THINK YOU'RE GOOD ENOUGH TO YELL AT ME, YOU BASTARD?!" Shinjuro began screaming like a madman, grabbing his son's wrist to keep him from fleeing. "YOU COULDN'T EVEN BECOME A SLAYER—HOW DO YOU EXPECT TO BECOME A HASHIRA?"
"F-Father, I swear I didn't mean to yell at you." Senjuro was terrified; he had never seen his father this angry, never felt such fear—especially caused by his father—and the sharp pain in his head made it impossible to calm down. "P-please, let me go."
"I'M NOT LETTING GO, YOU DAMNED WRETCH!" Shinjuro kept shouting, squeezing Senjuro's wrist so hard the boy wept from the pain. "I DON'T EVEN KNOW WHY YOU'RE STILL ALIVE!"
"D-Dad, you're hurting me!" Poor Senjuro tried to break free from the larger man's grasp as his father tightened his grip. "Dad, stop!"
"I WISH YOU WERE THE ONE WHO HAD DIED!!!" he shouted, leaving the boy stunned—had he heard that right? He finally managed to tear himself away from his father's hold, his hand aching and possibly broken. "Your birth was the primary cause of your mother's death—I've always thought so, and it's true. And now, Kyojuro died because he wasn't prepared for battle—because he was distracted training *you*. So, why don't *you* just die?"
Senjuro knew his father was right; his birth was the reason his mother had fallen ill and eventually died. His brother had been the one to teach him Flame Breathing, and because of that, Kyojuro had missed training sessions and grown weaker. Had it all been his fault? After hearing those words from his father, Senjuro ran to his room and shut the door behind him.
"...Pfft, he deserves it," the father muttered before heading to bed, seeing that night was falling.
Night had fallen; everything was dark and silent. One of the Rengokus was fast asleep, but the other lay in bed, wondering if sleeping forever might be a good idea. After all, he didn't think his father wanted his company—he never had. The best option, he decided, was to go to where his mother and brother were resting. That was his plan.
He hadn't been able to sleep soundly for many nights; it had started the day he was told of his brother's death. Since then, he felt as though someone were watching him—a sensation that only occurred at night. The feeling of being observed was terrifying, yet he also found it strangely comforting, as it made him feel less alone; the only reason he couldn't sleep was the paranoia of not knowing who was watching him.
But then, his thoughts were interrupted by a faint tapping at the window. He peered out to see who it was, only to be surprised to find no one there—just a spiderweb holding a note written on a leaf. It read: "Come to the forest; do not be afraid. I am here to help you."
Senjuro didn't hesitate to decide he would go; after all, if he died, there would be no one to miss him. He slipped silently out of his room and cracked open his father's door to check if he was still asleep. Seeing that he was, Senjuro grabbed a katana; even though he couldn't use a Breathing Style, the weapon would allow him to defend himself if the stranger attacked.
He left his home and headed toward the forest. It was quite late, but he hurried to get there; he wanted to confront the stranger face-to-face and ask how they could possibly help him when they knew nothing about him or the life he was living.
He paused for a moment. "It wouldn't be a bad idea to buy a rope," he thought; if the conversation with the stranger came to nothing, he could end his life in the forest. That way, when someone found his body, it would tarnish his father's reputation—he liked the plan.
Once in the forest, he waited for someone to arrive—someone who might tell him not to waste his life, to come live with them, or to run away together. But no one came; more than half an hour had passed, and Senjuro was losing patience. He began walking toward a tree, attempting to hang the rope from a branch, when he felt a stone being thrown at him. He turned instinctively, but he never expected what he was about to see.
"So, have you decided to end your miserable life?" a voice said, emerging from the darkness.
The person speaking wasn't human; it was a demon. Its skin was as white as snow, and its hair was just as pale; its visible eye was a very light blue, and it wore a white haori decorated with spiderwebs and red dots. Its mere presence made Senjuro tremble; one didn't need to be a Demon Slayer to know that this demon was strong—incredibly strong.
"You... you were the one who asked me to meet here, right?" he asked, nervous about the situation.
Senjuro didn't know how to feel—sad, scared, angry, or perhaps even happy? The person who had summoned him to the forest was a demon, and surely, it had brought him there solely to kill him. Seeing that the demon didn't answer, he assumed he was right.
"If you've come to kill me, just get it over with; I won't give you any trouble."
"I didn't come to kill you," the demon said curtly in response to the human's words. Wait—had he heard right? A demon hadn't summoned him to eat him? That was supposed to be impossible; from what he’d heard, they were beings who thought only of eating and growing their power. So, if he hadn't brought him here to eat him, why had he brought him?
"I haven't come to kill you; I'm here for something else."
"For what? Even if you brought me here for another reason... why me?" He spoke, still unable to grasp the situation.
He didn't understand it; he really didn't know why *him*. The demon could have chosen anyone—like that young man with the scar. He had talent and, above all, was a Demon Slayer—something Senjuro could never be.
"If it's not to eat me, I wouldn't really be of any use to you. I don't have a Slayer's abilities. And if you try to use me as a hostage, it won't work; I don't matter to anyone—no one close enough to notice I'm gone."
Did the truth hurt? Of course it did. But it was better to be honest and keep the demon from getting false hopes about attacking a Slayer.
"That's not why I'm here. I want you to be part of my family."
What? Senjuro’s mind went blank at the demon's remark. Be part of his family? Did demons even *have* families? He wasn't sure, but now that he thought about it... wouldn't it be easier to ask another demon to be his family? These and other questions swirled in his mind, but one specific doubt stood out.
"But... why me?" he asked, bewildered.
"I've been watching you, and I'm aware of your family life. It isn't exactly great, is it?"
It was impossible to tell from his tone whether he felt pity or mockery—but Senjuro didn't like either option. "So, why not start from scratch and have a different family? People who truly value and love you—that would be amazing, don't you think?"
Everything suddenly clicked in Senjuro's mind. The feeling of being watched night after night—had *he* been the cause? Now he knew, though he still didn't fully understand why *he* was the one being targeted.
"I want to build a new family, and you have a terrible one. Don't you think that if we joined forces, we could help each other?"
"I... I don't think I could do that. I would disgrace my family by becoming a demon, and that’s the last thing I want."
That act alone would be enough to bring disgrace upon his family; on top of his inability to use Flame Breathing, becoming a demon would be the final straw.
"I’m asking you nicely—please. And if you wish, I can give you enough power to finish off your father," he said, dropping the proposal.
Silence. He hadn't expected that offer. Kill his father? He had never planned on that. Suddenly, memories of his father’s abuse came flooding back. Did he want revenge? Of course—but he hadn't thought *this* was the right solution. Yet now, he found himself more than willing to become a demon. If he was going to be a disgrace, so be it—but he would have the power to take revenge on those who had hurt him.
"If you don't want to, I won't force you. I only ask that you don't reveal that you managed to see me..."
"I accept—I'll become a demon," he said, agreeing to the proposal.
This answer surprised the demon; he had expected young Rengoku to refuse, but it turned out he was wrong. "I only ask that you give me the strength needed to kill my father," he said, casting a cold glance at him.
This only heightened the demon's surprise; he never imagined Senjuro would agree—and consent to killing his own father.
"All right, I’ll turn you into a demon," he said, pulling out a small glass vial containing a red liquid. "This is Master Muzan’s blood. He gave it to me after I told him there was a chance I could turn someone from a lineage of Demon Slayers into a demon. You have to drink it." He held the vial out for Senjuro to take but quickly pulled it back to add one last thing. "Oh, and once you’re part of my family, you’ll help me catch the Slayer who wears hanafuda earrings and his little sister."
"Wait—you want to catch Tanjiro?"
Senjuro thought it over. Capture someone who had tried to help his family situation? He truly felt sorry for the boy and didn't want to lead him to a likely death, but he had already made the deal with the demon. After a moment of reflection, he spoke with absolute certainty.
"All right, I’m willing to catch him," he finally said.
"Good. Here, take it then," the demon said, handing him the vial. It wasn't even half-full; it looked like a very small amount.
"So, if I drink this, I’ll turn into a demon?" he asked as he took the vial. The demon nodded, but Senjuro paused to think: if this was Muzan’s actual blood, drinking it might kill him. "Hey, this isn't going to kill me, is it?"
"No, you fool. Muzan’s blood *can* kill people, but that only happens if too much is used at once. Since this vial isn't even half-full, it’ll turn you into a demon without any issues." He said this with such confidence that Senjuro felt reassured enough to almost start drinking—but then, an idea struck him. "Hey, wouldn't it be better if we didn't leave any trace that I turned into a demon? That way, it’ll be easier for me to catch him by surprise."
He said this while unsheathing his katana. The blade didn't change color when he touched it—a detail that brought a pang of sadness to the young boy.
"That would be best," he said sincerely.
A surprise attack would give them a better chance of capturing him. Besides, knowing Senjuro from his human days, he knew he couldn't bring himself to cut off the boy's head; he was too sentimental—which was exactly why he wanted him in his family, alongside his sister. But he snapped out of his thoughts when he saw Senjuro unsheathing his katana.
"What are you doing?" He didn't know what the young Rengoku intended, but he watched as the boy placed a small piece of cloth in his mouth and, with a single, swift stroke, severed his own arm while clenching his jaw. "But... what the hell are you doing?"
"I-if we want them not to know I turned into a demon, the best idea is for them to think I was eaten," he said, fighting back the urge to cry. He knelt on the ground and continued, "We'll leave it here; that way, they'll think I was devoured and that my arm was the only thing left of me."
After speaking, he opened the bottle and took a drink. He felt his entire body—his appearance, his genes, everything—undergoing a transformation. Yet, he felt no pain; it felt strange, certainly, but it didn't hurt—perhaps because he had chosen to become a demon himself, rather than being forced into it. After a few minutes, Senjuro’s body had transformed completely; like Rui, he now had red markings on his face—which had turned stark white, matching the rest of his skin and hair—though he still retained certain traits from his human form, such as his hairstyle and its red tips. His arm had regenerated, and, gripped by an overwhelming urge to consume flesh, he bit into the first thing he found: his own arm. He took only a single bite before looking up to see the demon—whose second eye was now revealed, bearing the kanji for the number five, marking him as a Lower Moon. Senjuro finally understood the source of the demon's immense strength, yet he saw the creature extending a hand, inviting him to take it.
"My name is Rui. Welcome to my family, Senjuro," he finally said.