[Kimetsu no Yaiba] a promise for another day

Title: a promise for another day
Fandom: Kimetsu no Yaiba / Demon Slayer
Pairing: Rengoku Kyoujuro / Kamado Tanjirou
Words: 1,422
Completed: 2023/05/29
Summary: “I love you, Sensei.”

The confession was unexpected to say the least, both for the recipient as well as for the person who uttered the words. One moment Tanjirou was asking Rengoku-sensei something about the previous class – at which point the pair exchanged some light banter – and the next moment his mouth was acting faster than his brain had the chance to react and stop him.

Links: AO3 & FF.net

“I love you, Sensei.”

The confession was unexpected to say the least, both for the recipient as well as for the person who uttered the words. One moment Tanjirou was asking Rengoku-sensei something about the previous class – at which point the pair exchanged some light banter – and the next moment his mouth was acting faster than his brain had the chance to react and stop him.

Maybe getting out of the situation would be a lot easier if the ground swallowed him up, but alas mercy wasn’t on his side today and two fiery orbs looked at him in shock as if he had just grown a second head. At least his teacher didn’t appear to be disgusted with him – which seemed to be the only blessing in disguise here – otherwise Tanjirou would never have been able to live with himself.

“I… I m-mean…”

Taking back the words was not an option, and apologizing was equally out of the question. Yes, Tanjirou was selfish enough to unload his feelings on his teacher, but apologizing would mean downplaying his feelings, belittling them almost as if he were ashamed to feel the way he did, or even calling them a mistake. But how could this affection, this deep longing for the man before him ever be wrong or a mistake? No. And for that very reason, Tanjirou would hold his head high and accept the response to his blurted confession as gracefully as could be expected from a teenager his age.

(That is, he’d cry as soon as he was alone – a lot – and later call his friends to be comforted, since they’d have his head if he didn’t check in with them.)

“You…” Rengoku-sensei seemed to have regained some of his composure and he huffed a chuckle that sounded unusually shaky for the otherwise confident teacher. “You couldn’t have just waited three more months, could you?”

The boy’s eyes grew wide in surprise and he racked his panicked brain as best as he could to follow his teacher’s words. Three months, he had said. What was in three mo–- oh.


“…Gra-” Tanjriou swallowed, and something as treacherous as hope blossomed in his chest, “graduation…?”

“That’s right, my boy.”

Rengoku-sensei had never called him ‘his’ before. It was always Kamado this and Kamado-boy that, but never truly his, and Tanjirou melted over those words and their implication. It must have reflected on his face because the older man chuckled (full of mirth this time) and Tanjirou could swear that his face not only resembled a tomato but was hot enough to melt butter.

“You’re really putting me on the spot now,” Rengoku admitted, opening the drawer of his desk and rummaging around inside. The item he was looking for seemed to be well hidden, and the teacher let out an irritated sigh. Not wanting to disturb the older man, Tanjirou watched in silence and was startled by the excited ‘Ah!’ his teacher belled without warning.


Rengoku’s voice boomed, and Tanjirou tensed reflexively.


“Give me your hand. Your left one specifically.”

Certainly an odd request, but not one that the student was about to question, for he trusted his teacher blindly. Somewhat confused, Tanjirou obeyed his teacher’s plea and extended his arm. His breath caught as Rengoku grasped his hand gently, almost as if he were holding something precious. More than once, the boy wondered what his teacher’s touch would feel like. He had always associated the man with warmth and fire – if his passion for teaching his students, which burned so fervently in his eyes was anything to go by – and Tanjirou felt a tender and eager heat flow over his body, fierce enough to make his knees weak.

“As much as I wish to, I can not return your feelings right now.”

Rengoku-sensei opened his right hand and the boy finally caught a glimpse of the mysterious object. It was a woolen thread, probably about 5 or 7 centimeters long, but in itself nothing special.

“Certain… boundaries are restricting me at the moment.” The older man chuckled, trying his best to hide his nerves, “You know, I have been telling myself day in and day out to be patient and wait for March to come, but you just had to barge in and thwart my plans, didn’t you?”

Wait a minute.

“Nevertheless! Your courage and candor should not go unrewarded.”

Now just hold on for a second.

“I want to make you a promise that you can take with you all the way to graduation.”

Tanjirou’s mind was racing, barely able to keep up with the thoughts running through his head. There… there was no way! This could not be! But what if… no, no! He must have finally fainted out of embarrassment because there was no other explanation than that he was just experiencing a very realistic but cruel fever dream.

“A promise,” Rengoku continued, oblivious to his student’s inner turmoil and tying the thread around the boy’s finger just tight enough to prevent it from slipping off, “for you to wait for me and see me again in about three months’ time.” He gently squeezed the boy’s hand and slowly, almost afraid to break the spell, brought it to his lips. Tanjirou was sure that time stood still at that moment, and all his senses focused on the tingling on his skin left by his teacher’s hot breath. Neither of them could tell how much time had passed, but finally, Rengoku let go of the boy’s hand before he could close the gap and smiled at the student instead.


There was so much more Tanjirou could perceive in Rengoku-sensei’s scent and it overwhelmed the boy, taking his breath away. The student fixed his gaze on his left hand, where a thread of red wool he had thought nothing special of only minutes before was now tied around his ring finger. A promise, Rengoku-sensei had called it, and all at once the hidden meaning hit the boy, almost knocking him off his feet and making him gasp for air.

“Sen–” His eyes were burning, a treacherous telltale sign of the tears to come and Tanjirou scolded himself not to cry, no matter how much the relief he was feeling gave way to tears of joy and happiness. “Sensei…”

“Boy, you have no idea how much I want to kiss you right now.”

This confession alone was enough to break the dam of tears. Startled by his outburst, the young man inhaled sharply and frantically tried to dry the tears with the sleeve of his blazer.

“I love you…” the boy murmured in between tears, the sleeve barely muffling his voice, “I love you. I love you so much, Sensei…”

Rengoku reached for the boy in front of him, but paused midway, arm hanging awkwardly in the air, before mustering the courage to run his hand through the mop of burgundy hair, soothing at first, but the motion quickly gained momentum and soon he was ruffling the boy’s hair until it resembled a bird’s nest.


It was still a gesture innocent enough to be shared between teacher and student.

“It’s getting late now, boy. You should go home.”

Rengoku wasn’t fooling anyone with this obvious lie, but he was offering them the perfect opening, the perfect excuse to part ways before one of them physically crossed the carefully constructed but alas thin line of teacher and student they share. For now, this brief moment of honesty and confession they allowed themselves would have to be enough.

“Um,” Tanjirou agreed and nodded his head. The student was aware that as soon as he left the room, the magic surrounding them would disappear and he wouldn’t be able to hear his new favorite pair of words (‘my boy’) for a while. Perhaps that was exactly why Tanjirou kept repeating them over and over again in his head, bathing for as long as possible in the feelings those words evoked in him.

“Oh, and Tanjirou?” The boy paused, one hand already on the doorknob as he turned to face his teacher and nearly drowned in the wave of affection emanating from the man. “I can’t wait for you to graduate.”

As the door closed behind Tanjirou and the boy found himself alone in the hallway, he brought his left hand up to his lips and kissed the red thread on his ring finger.

“I promise, Sensei,” he vowed, “so please, be ready for me in three months.”

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.