I published this on AO3 a while ago, but just now am getting around to adding it here. Oh well. A while back, this concept drilled its way into my head and didn't stop until I wrote it down, so here we are.
Due to how similar some of the names in this one-shot are (Seriously guys, you've got one letter difference between father and son!), here's a little cheat sheet:
Tanjiro: Or favorite older brother
Tanjuro: Sick dad
Kyojuro: Flame Hashira
Senjuro: Flame Hashira's little brother
Shinjuro: Flame Hashira's alcoholic dad.
He only thought about his old battle-brother again after nearly a decade due to Kyojuro. The boy had gotten it into his head to succeed his father as the Flame Hashira, despite his own lack of talent and Shinjuro's despair. Kyojuro stole the flame breathing books of his ancestors and was still training in secret. This led to a loud argument that only ended when Senjuro - timid, quiet little Senjuro who usually hid in a corner - jumped on Shinjuro's back to try and pull him away from his brother.
Once Shinjuro had settled down and made it to the bottom of a sake bottle, he realized that it wasn't his son's fault that he was so impertinent. Tanjuro had retired when Kyojuro was still a baby, so he couldn't remember what true greatness was. Most of the Demon slayers from that time were dead now. Of those who had fought beside the Sun Hashira, only Urokodaki, Old man Kuwajima, and Shinjuro himself remained in the land of the living. Even the late Ubuyashiki head had finally succumbed to his curse and left matters in the hands of young Kagaya.
It had been so long. Over a decade at this point; they were in the Taisho Era now. It was past time for Shinjuro to visit his battle-brother and possibly forgive him for leaving.
"Excuse me, but do you know Tanjuro Kamado?"
"Hm?" The shopkeeper in the small village town tilted her head. "Oh, why yes! Kamado-san makes the best charcoal in the prefecture. And his family is so kind! I wish my little Kanime would take after Tanjiro, you rarely see such a well-behaved boy these days."
So, he does have a family. "I'm an old acquaintance of his and haven't visited in a while," Shinjuro carefully explained. "Would you mind giving me directions to his house?"
"Of course!" The lady clapped her hands together. "Just follow the mountain pass over there, past Saburo-san's house, for about half a day. Actually, Tanjiro-kun just left here, so if you run you may catch up to him.
Unlikely, especially if he's from that man's bloodline.
"You look well, Rengoku."
Shinjuro couldn't bring himself to reply with the same greeting. When he'd last seen Tanjuro, the only sign of his debilitating illness had been a frequent cough. Now, the man's face was hollow, all his muscle tone was gone, and those eyes that once burned with the sun had all but lost their light. According to Kie, her husband couldn't even walk more than a few steps outside without assistance.
An angry part of Shinjuro wished that he hadn't come, so he could only remember his battle-brother in his prime.
"...It's been too long." Shinjuro finally said, sitting down on the porch next to Tanjuro.
"How is your family? Are Ruka and Kyojuro doing well?"
"...Ruka passed away several years ago. She did give me another son, Senjuro."
"I'm sorry to hear that. Still, I have no doubt that your sons' are a credit to her memory."
"Everything that's good about them came from her."
Tanjuro sighed. "You've always been so hard on yourself, old friend. You may be the eldest son, but you shouldn't try to carry the world on your shoulders. We are all only human."
"Some of us are," Shinjuro muttered.
"Please, not this old argument again." No matter how many times Tanjuro tried to assure him that there was nothing inherently superior about the breath of the Sun users, Shinjuro refused to hear it.
A young, miniature Tanjuro ran up to the two men. "Father, will you be okay with Hanko and your friend while I help Nezuko and Takeo with the wood?"
Tanjuro smiled at his son. "We will Tanjiro. But before you go, would you mind showing me your Hinokami Kagura? I want to make sure you're practicing."
Shinjuro gasped and turned to his friend.
Tanjiro, for his part, looked unsure and cast a glance in Shinjuro's direction.
"It's fine," Tanjuro assured his boy. "Shinjuro is an old friend, he's seen me do that dance many times."
This was apparently enough for the boy, who nodded, ran off to the edge of the clearing, and picked up a stick to serve as a substitute for the blade.
"So, at least the legacy of Sun breathing will continue on." Shinjuro muttered.
Tanjuro only grunted.
Tanjiro moved through the set styles hesitantly, with shaking arms and unsteady feet. Still, Shinjuro could already tell that the boy would master it eventually. He may not be at the level of his father, but that boy would surpass anything Kyojuro could accomplish in no was surely blessed by the Kami.
"That boy will be a great demon slayer someday."
"No."
The response was so sudden and unusually fierce that Shinjuro originally didn't realize that it was Tanjuro speaking. "What do you mean?"
"Tanjiro won't be a demon slayer. I want him to live a peaceful life, unconcerned with those tragedies. I want all my children to live long, simple lives."
"You can't be that naive!" Shinjuro shouted. "That boy has the mark!"
"You're wrong. That scar on Tanjiro's forehead is from when he saved his younger brother. Besides, I doubt that even I have the mark you are looking for. If what the records say is true, those around me should have achieved the mark as well, and none of you did. I for one, am glad for that. You've passed the age of 25 already, and I'd hate for you to not see your sons grow up." At the end of his speech, Tanjuro's voice broke into coughs.
Tanjiro noticed his father's state and ran up to them. "Father! Don't exert yourself!"
Shinjuro stood up. He looked down at his old friend, his battle-brother, the man he admired most, and was disgusted. The Sun Hashira was reduced to an invalid, and his chosen successor had the temperament of a nursemaid instead of a warrior. It was pathetic.
"Coming here was a mistake." Shinjuro said. "I'll take my leave now."
He would eventually regret that those were the last words he said to his old friend
Unknown to Shinjuro, his visit did have an effect on Tanjuro Kamado. That night, he pulled his eldest son aside and showed him a Nichirin blade.
Tanjiro's eyes sparkled in wonder at the blade. "Father, are we from a family of Samurai?"
Tanjuro chuckled. "No, nothing like that. You may see this as a family heirloom, but it was only forged in my generation. We are a family of charcoal-sellers, after all."
Tanjiro nodded. He looked a little disappointed, but he was a child of the new Era and didn't need to worry about legacies from the Edo period.
"Tanjiro, as you are the oldest son you will probably inherit this house once your mother and I have passed on. You will have a new family to care for, and will continue our traditions. However, if the day should ever come when you or your descendants need to leave this place and face great danger, I ask that you please take this sword with you. It is strong and sharp, and you can protect yourself and others with it."
Tanjiro would remember these words before he left for Mt. Sagiri with his sister, and would carry it to Urokodaki's house though it never occurred to him to unsheathe the blade.
Several years later, Kyojuro came home and announced that he was the new Flame Hashira. Like that was any great accomplishment. Shijuro became frustrated with his sons, downed another bottle of sake, and decided to do the stupid thing and visit Tanjuro again.
This time, he remembered the way and didn't need to stop by the village. If he had, he might have noticed how sad they were at the mention of Kamado and might have learned the truth earlier.
Instead, he made it all the way to that little house on the mountain before he saw the graves.
All he could do was pay his respects. Someone had already cleaned the house, but based on the broken door and family history Shinjuro could easily guess how they'd died.
The whole time he stood there, one question ran through his mind: What could I have done to prevent this?
"Kyojuro said you wanted to speak with me?"
Shinjuro turned to look at the young man in his presence. The current Water Hashira, Giyu Tomioka, was not an intimidating man. His skills were certainly a testament to Urokodaki's tutelage, and he may have somewhat surpassed his old master, but he was like water. Calm and unemotional, but ready to flow through the path of least resistance.
He certainly did not have the skills to combat someone even a Sun breath user couldn't defeat.
"I have an old friend who lives in your domain…" Shinjuro described the path of the Kamado household, or at least what was left of it. "...I recently went to visit him, but I found only an empty house and buried graves. I suspect they were killed by a demon. Did you ever run into any demons in that area?"
Tomioka stood there silently for about a minute. Shinjuro got frustrated and started to get up and leave. If the man didn't know who he was talking about, then there was no point in talking to Tomioka any more.
"...Kamado. That is your friend's name, isn't it?"
Shinjuro froze in a half-kneeling position. "Yes."
"I remember them. It was a little over a year ago now. I received a notice that there was a strong demon in the area, but by the time I got there everyone in the house was already dead. I'm sorry. If I had made it there half a day earlier, I might have been able to save them."
Shinjuro leaned back again. He couldn't bear to think that the legacy of Sun breathing was truely dead. He certainly couldn't bear to think of Tanjuro's children being brutally massacred. Still, he couldn't blame the Water Hashira for this.
Tanjuro's words rang in his head. We're all only human.
"You're only human. It can't be helped."
"...You should know that one of them escaped unharmed. The oldest son was staying in another house that night and wasn't attacked."
Shinjuro sat up at that. "The eldest is alive? Tanjiro, right?"
Tomioka nodded. "He had a strong will and showed great battle instincts, so I sent him to my old master Urokodaki to be trained. I suspect my master wouldn't send him to Final Selection this early, so he's likely still there if you want to learn more about what happened to your friend.
"... I guess I'll have to visit Mt. Sagiri." Somehow, he doubted Urokodaki would know who he was working with, even if he'd been acquaintances with Tanjuro.
The hike to Mt. Sagiri was hell on Shinjuro's gout-ridden joints. He was getting too old for all this traveling. Still, he owed it to Tanjuro to check on his son's progress, and he wouldn't be able to rest until he was sure Sun breathing was being used again.
When he finally reached that little house at the base of the mountain, the only one waiting for him was Urokodaki.
"Giyu sent a letter after you spoke with him, Rengoku. I suspected you'd come eventually."
Shinjuro snorted at that and sat down on the floor. "Have you got anything to drink?"
"No, unless you're referring to tea. Why are you concerned with Tanjiro Kamado?"
"You may be an idiot, but you're not that blind or dumb. His name is Kamado." Shinjuro sighed. "He's the son of our Tanjuro."
"And what does the identity of that boy's father have to do with anything?"
Shinjuro balked. There were no words for how stupid Urokodaki was acting, so he just glared.
Urokodaki sighed. "You know, when I finally realized where I'd seen those hanafuda earrings before I was tempted to send for you. You were Tanjuro's best friend and should have been the one to guide his son. Now, I'm glad I didn't. Kuwajima at least took a moment to mourn our old friend before asking if I thought Tanjiro would survive final selection."
This infuriated Shinjuro. "Who do you think - "
"Urokodaki-san!" a young voice called out from beyond the doorway. "It's getting dark. Is dinner…" He froze when he caught sight of Shinjuro.
"Tanjiro-kun, this is an old friend of mine, Shinjuro Rengoku." Urokodaki said. "Please forgive his intrusion."
"You… I remember you." Tanjiro said. "You came to visit father years ago. How do you… how do you know both my father and Urokodaki-san?"
"Hm." Shinjuro grunted. "I heard about what happened to your family. You have my sympathies."
"Ah, thank you." Tanjiro finally entered the hut and sat down.
Shinjuro scrutinized the boy critically. He had grown a great deal in the last few years, and had finally developed some muscle tone. It seems Urokodaki's training was good for something at least. Tanjiro had also lost his child-like innocence. There was steel in his soul, and he had the eyes of a warrior. Just like Tanjuro used to.
"Tell me boy, do you remember your father's Sun Breathing?"
Urokodaki sighed in exasperation.
"Sun… breathing?" Tanjiro looked at the other two men in confusion.
"Come on, I saw you do it when I last visited your father." Shinjuro waved his hand. "He said his usual nonsense about it being a prayer to the gods again…"
"Are you talking about the Hinokami Kagura?" Tanjiro asked. "Are you saying… that it's actually a sword style?"
Both Urokodaki and Shinjuro stared at the boy in shock.
Shinjuro recovered first. "Yes exactly."
"But… father never mentioned…"
"Tanjuro retired from the demon slayer corps before you or your siblings were born." Urokodaki said. "I imagine he didn't want to pressure you to follow a path he knew was fraught with danger and would lead to an early grave."
Shinjuro rolled his eyes. "Fat lot of good that did him."
"Don't talk about my father like that!"
Even Urokodaki was surprised by Tanjiro's outburst.
"All my life, Father had a frail body. By the end, he couldn't walk on his own and could barely get out of bed. Still, he took care of us the best he could. And every new year without fail he'd dance from sunset to sunrise nonstop! So don't disrespect him!"
Shinjuro was shocked to notice that Tanjiro was starting to cry.
"Father… father had passed away several months before the attack. I wasn't there, I was peacefully sleeping in another house while my family was being brutally murdered. Still, despite my own regrets, I know that the only person responsible for their death is Muzan Kibutsuji! That's why I decided byself to become a demon slayer! For their sake!"
Tanjiro was standing up by the end, breathing heavily.
All three occupants stared at one another for a long while, before Urokodaki finally broke the tension. "Rengoku, it's dark out so I'll let you stay the night, but you should leave tomorrow morning."
Shinjuro scowled. "Yeah. I can see that I'm not wanted."
That night, while Shinjuro slept in a spare room and didn't wonder about the closed door nearby, Tanjiro spoke to Urokodaki about his father. For the first time in his life, he learned about how great of a swordsman Tanjuro Kamado had been. How he had risen to the rank of Hashira and killed hundreds of demons in his short tenure with the corps. How he was the man both Urokodaki and Shinjuro admired most.
When Tanjiro finally remembered his father's sword, he asked for permission to train with it. Urokodaki granted it without a second thought, though he knew the requirements for breath of water sword was slightly different from breath of sun.
"Urokodaki-san, did my father ever battle Muzan directly?"
"No. None of the demon slayers have even seen him in centuries. But, if there was anyone who had a chance, it would have been your father. He slayed 4 different lower moons over the course of his career, and even battled against Upper Moon 3 and survived until they fled with the sunrise."
Right after that night, Tanjiro started training to use the Hinokami Kagura beside his breath of water. It was difficult, especially since the spirits of dead children could only help with the breath of water, but he was able to split the largest boulder within a month, half a year earlier than he needed to qualify for the next Final selection.
Tanjiro would feel guilty about getting a new Nichirin blade after final selection when his father's was perfectly adequate, but when Haganezuka-san was so excited to see how the blade would change color Tanjiro decided to use it for a while. At least, until it broke at Mt. Natagumo and he felt better just asking one of the swordsmiths to sharpen the older blade.
"Come Father! Come meet my three new tsugukos!"
Kyojuro was as loud as ever. Subtlety was never the boy's strong suite, and bursting his eardrums years ago hadn't helped matters. At this point, talking with his son was exhausting for Shinjuro.
"What makes you think you have anything to teach these tsugukos? I heard about your last mission. You're now blind in one eye!" Shinjuro grumbled.
Two new voices rang through the Fire estate.
"WHAAA-"
"Oi! What are you saying about Rengacho? I'll fight you!"
The most striking interruption though was a streak of red that rammed into the back of his head.
"Don't belittle Rengoku-san!"
Shinjuro rolled off the porch and into the garden, finally landing on his back. The blinding high-noon sun didn't help his hangover and budding concussion. It was almost a relief when a figure blocked the light, until he realized who that red hair and dangling earrings belonged to.
"Flame Hashira Kyojuro Rengoku is a magnificent swordsman! He protected five train cars by himself when we were fighting the Lower Moon One! When that was done, he immediately fought with Upper Moon Three and survived! Sure he lost one eye in the battle, but that hasn't diminished his fighting spirit!" shouted Tanjiro Kamado.
Shinjuro couldn't do much more than blink. "... Kamado? Is that you?"
Tanjiro turned away and bowed towards Kyojuro. "Kyojuro-san, please forgive me for being so disrespectful to your father. However, I couldn't stand by and let this man who claims to admire my father speak so ill of you."
"Ha! That is no problem, it's about time someone gave him a good head-but." Kyojuro laughed. "I only hope your head isn't hurt too bad as a result."
"Nope! I have a very thick skull!"
"Ha ha! Oh, you mentioned your father, Tanjiro-kun. Is that who you learned Sun-breathing from?"
Tanjiro nodded. "Yes. I always knew it as the Hinokami Kagura, but after I started training with Urokodaki-san this man came by and mentioned that my father used sun breathing, and I started to incorporate it into my sword style as well."
"I see. My father frequently mentioned his old friend who practiced sun breathing, but I never had the pleasure to meet him. Still, this is wonderful! Perhaps your ancestry is responsible for your sister's unique condition."
"That's what Urokodaki said as well!"
As Tanjiro and Kyojuro laughed and talked, Shinjuro couldn't do much more than sit up and look at them. Ignoring the blonde and boar-head in the background, the sight before him was like a blast from the past. Kyojuro and Tanjiro, they were just like Shinjuro and Tanjuro, only better and more at ease.
Kamado, old friend, it seems our sons have surpassed us both.
Taisho Secret: Giyu took so long to respond to Shinjuro when they were talking about the Kamado family because he wasn't sure if he should mention Nezuko. In the end, he decided to keep quiet and leave it to Tanjiro to decide. Between this and Rengoku stubbornness, Shinjuro didn't find out about her until after that last scene.
I'm not quite sure what butterfly effect would have led to Kyojuro surviving in this AU. Either Tanjiro handled the upper moon one easier and was still in shape to help with the fight or Akaza took one look at Tanjiro, had flashbacks to fighting his father, and ran the hell out of there as fast as he could. It was probably a combination of both.
Honestly, I feel like this concept deserves a multi-chapter fic or at least a prequel, but I don't have the energy, time, or drive to do that kind of thing right now. I'm only writing this to make myself feel better before trying to jump back into other projects. So please, take this as an invitation to take this concept and run with it. I'd appreciate it if you linked to this story as inspiration, but that's mainly just my ego talking.