Hello! This is my first Kimetsu no Yaiba fic! I love Yushiro, not just because of how salty he is but because he shares the same voice actor as the cinnamon roll known as Midoriya Izuku in Boku no Hero Academia! Yamashita Daiki is indeed a cinnamon roll himself and he's super talented along with many other voice actors.
Before reading, please take note of my headcanon: After Tamayo's death, Yushiro reviewed her notes on demons. Like Tanjiro said, he doesn't intend to die so he could continue to remember Tamayo but I imagine he's grown tired of not going into the sun and having to drink blood. Eventually, he's able to gain strength through sleeping instead of drinking blood and can step into the sunlight but not for too long.
Another note is that I am aware that demons can die. Not easily, but they can. However, I don't think Yushiro is the type to keep running into danger especially if the war between demon and humans. The Demon Slayer Corps has disbanded and there is no one to hunt demons anymore. Even if a random mugger stabbed him, he could survive something like that. As far as I am concerned, he will continue to live forever.
Third, this takes place before the last chapter (modern day) and so he's not an artist yet. I have a headcanon that painting is something that he picks up abroad and when he returns to Japan, he expands on it. Eventually, he makes enough money as an artist and retires from being a doctor though he still writes a lot of medical journals.
Chachamaru was the first to die. Of course, it made sense considering that cats have a short life span even compared to a human. Still, it was a shock to Yushiro when the feline lay its head on his lap for the last time, closing its eyes for the last time. Even with all the best medicine, there was nothing that could be done about the cat's old age.
Yushiro stood in his backyard with the rain soaking his clothes as he dug a grave for his dead companion. Tears ran down his face but he refused to acknowledge them, telling himself that it was just the rain. Demons didn't catch colds but he was tired and collapsed onto the couch after the impromptu funeral. He drifted off to sleep, something he had started doing after reading through Tamayo's notes about Nezuko.
Having falled off the couch, the floor was where Tanjiro found the demon, still soaked from the rain. He was sharply told to get out and the redhead obliged. However, he sent his sister, having picked up on the distress. Damn his kindness. At least Nezuko was much more bearable though he protested when she made him change into dry clothes and sat him down for tea.
"Where's Chachamaru?" Nezuko asked, looking around.
"Dead," Yushiro answered blankly.
"Yeah, cats die,"
It was just a cat. That's what he wanted to say to cover his emotions. It's what he wanted to tell himself so he didn't have to feel but it wasn't just a cat. He had lost a companion and the seat next to him on the sofa felt cold and empty. It was also a reminder that Yushiro, the last remaining demon, was possibly one of the last few immortal creatures in the world. It meant that he had to watch everyone around him die.
"You're going to die," he finally said.
"Yeah, I am," Nezuko whispered, knowing what he meant. "But when I die, I will still die a grateful person, indebted to you and Lady Tamayo,"
She was a ridiculously kind person and it really pissed him off but this time, he didn't feel any irritation. His chest still felt empty but he felt oddly calm. It wasn't for nothing that he had come to this point in his life. He had fought in a war, protecting humans from his own kind and with his allies, had succeeded.
"Thank you, Nezuko,"
Kiriya had offered Yushiro a place to stay but he had refused. Having faith that he wouldn't turn against humans, the Ubuyashiki family let him got but not without blessings. With his own two hands, he built a house near the former Demon Slayer Headquarters just far enough away from the wisteria. There was something in his heart that told him he needed to stay close. Tamayo had put her heart into saving this place and though he wouldn't admit it out loud, he would do the same if danger ever arose again.
He went to a university and earned a degree in medicine, a small feat for a former apprentice of a doctor. That was when he reluctantly agreed to deliver Nezuko's first child and became the unofficial family doctor for many former Demon Slayers. For a while, there were no deaths; only births. He wouldn't ever admit this out loud either but he liked children. As a human, he had disliked them but now, as he administered cold medicine for the little humans and scolded them for eating too many sweets, he felt a fondness growing.
It was then when he noticed that Kanao would run her fingers along faint crow's feet. She and Tanjiro had married and had children together and she was no longer a teenage girl. When she realized that Yushiro had seen her, her hand dropped and she gave an empty smile.
"Isn't it silly of me to be so vain about my fleeting youth?" she bowed her head in shame.
"No, it's just how mortals are," Yushiro shrugged, not really caring.
"I guess I'm just afraid that maybe Tanjiro will stop loving me if I look older,"
"You're an idiot if you really think that. As sick as he makes me, he's not the type to love someone for their looks,"
That was when he realized that after a certain amount of time, he would have to relocate. People liked to talk and they would certainly talk about the man who didn't age. He certainly didn't want that kind of attention; he wanted to live the peaceful life that Tamayo had sacrificed herself for.
"You'll always be in our hearts," Kanao smiled knowingly.
"Well, if you keep bothering me, I'm sure I will be,"
Leaving the Kamado household, he felt noticeably lighter than he did that morning.
If there was one person that Yushiro came the closest to liking, it was Giyu. He was quiet, quick to the point and never stuck around unneccessarily. He showed fondness for others, having become happier after the war but around the demon, he was quiet. Perhaps he sensed that Yushiro wasn't the type to waste time and was impatient so he made a point not to bother him.
When he grew sick, Nezuko dragged Yushiro to the former water pillar's bed to treat him. The demon did not say anything and just rolled up his sleeves, reaching into his medical bag. He sorted through various medical instruments when Giyu stopped him.
"I'm old and I've lived my life," he sat up. "I'm going to die soon and no medicine in the world is going to help me,"
"Giyu-san! Please don't say that!" Nezuko teared up.
"My time is coming,"
Yushiro didn't answer and kept preparing his medical supplies when Tanjiro burst in. The older man repeated what he had said earlier and the redhead began crying with his sister. Having had enough, the demon turned around and yelled at the siblings to get out so there could be some damn peace and quiet. Giyu softly chuckled when the door closed and shifted in the bed.
"When I almost died, Lady Tamayo gave me a choice," Yushiro draped his stethoscope around his neck. "If I had refused, she promised to do everything she could to make me feel comfortable until I died,"
Giyu didn't answer and patiently waited for the other to continue.
"I intend to stay the last demon in this world but I can make your remaining days peaceful,"
"Thank you, Yushiro-san,"
And so, thanks to a certain demon, Tomika Giyu was able to laugh and spend time with his loved ones before he lay down to sleep for one last time.
He had opened his own clinic despite knowing that he would have to relocate in a few decades. Tamayo would've wanted him to continue helping people. The nearby village looked to him as their medical guru and though he was known to be a bit callous, he cured many people of their illnesses.
This was also the time that he fell in love for the first time. He had loved Tamayo but it was more of a parental love for his mentor. She had saved his life after all and he was still devoted to her but this girl stole his attention in a different way. The young woman named Mayu was a sickly patient of his and she often frequented his clinic.
"Doctor, I'm going to die," Mayu softly said.
"Of course, you are," Yushiro continued organizing medication. "Everyone dies,"
"No, you know what I mean,"
Slowly, Yushiro turned to look at the woman before him. She was the same age as Tamayo when she had died but of course, Yushiro was much older now. Her ebony black hair had a beautiful sheen to it and though she was a little gaunt, her skin remained smooth and pale. She was gentle and her laugh sounded like a pleasant chiming of bells. He knew that she was going to die soon and it drove home that he wouldn't die at all.
"Doctor, I love you,"
A silence filled the room and the demon felt his heart shatter into a million pieces. He already had enough (hidden) emotional ties to the Kamado, Agatsuma and hashibira household along with a few others. He couldn't afford to have any more attachments or he would just keep getting hurt again.
"I have a favor to ask of you though it may be selfish," Mayu continued.
All Yushiro could do was nod, trying to keep calm as his emotions surged through him violently.
"Will you please make love to me before I die?"
Bottles of pills clattered onto the floor and though they were thankfully closed so that their contents didn't fall out, he made no move to pick them up. He stared at Mayu who began to apologize but he stopped her by silently holding up his hand. It was embarrassing to think about but Yushiro had never know the intimacy of another person. Being involved in a war between humans and demons didn't leave much time in his life for that. An unfamiliar want tugged at him and after a moment, he lowered his hand and reached over to tuck a strand of dark hair behind the waiting woman's ear.
"I love you too,"
This death was bittersweet; it was inevitable but Yushiro made sure that she had the funeral fit for a queen. Her family had initially refused but he pressed on, grateful for the chance to know love at least once.
Tanjiro was the next to die and was shortly followed by his wife, Kanao. A decade later, Zenitsu followed with Inosuke dying a fews years after. Nezuko remained and became a frequent visitor to Yushiro's clinic. Her body was no longer young and her joints often betrayed her, creaking and paining the old woman. Though her children always insisted on escorting her, she would decline and happily chat with the demon doctor.
"Do you know where you're going to move to?" she asked one day.
It had been decades since Muzan had been defeated and people were beginning to gossip about the doctor's seemingly never ending youth. Kiriya had once again extended his offer for a place to stay but once again, Yushiro had declined. Still, if he didn't want to attract any unwanted attention, he needed to leave the area.
"Maybe you can visit another country!" Nezuko suggested. "See the world,"
"That's not a bad idea," Yushiro pondered. "Especially coming from you,"
"Oh, Yushi-kun! Please don't ever change,"
"I don't have much of a choice, you know,"
Nezuko laughed and her companion just rolled his eyes. However, travelling abroad did sound appealing. All his life, he had never left Japan and he supposed that there was so much more he could learn. He could continue to keep up with the latest medical discoveries and continue Tamayo's legacy.
"You'll live forever, right? Then you'll be Yushiro forever,"
"That's kind of obvious,"
"No, I mean that even if you're the only one who will remember Lady Tamayo after we're all gone, you shouldn't be just that,"
"I owe everything to her,"
"I know but you should be Yushiro as well!"
"Don't you have to get home before dark so you don't trip over your own feet?"
Again, Nezuko laughed and stood up to give a hug to her reluctant friend who returned it despite his protesting. The next day, he received a call to the Agatsuma household to find a lifeless yet smiling Nezuko laying in her bed. Afterwards, he didn't return to the clinic and instead, went home to pack his belongings. He bought a plane ticket to America and flipped through a book, refreshing his knowledge of the English language.
Of course, he planned on returning to Japan some day; that was his home and it was where he had live his long years so far. It was where he had met Lady Tamayo and many others who had played important roles in his life. He knew that Japan would be different by the time he came back but he wouldn't change. After all, he would be Yushiro forever.