Requested by @uwukoyuki
(h/l): hair length
(h/c): hair color
(e/c): eye color
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It was the hour of sunset when Kokushibou ventured into the forest. This was the norm for him, but it wasn't to get a head start at night. Ever since he had become a demon, he had always gone out to stargaze before tending to his nightly duties be it feasting or searching for the Blue Spider Lily. It wasn't long before Kokushibou came upon the cliffs where he stargazed. He had a deep sense of loneliness while stargazing. A woman's voice also echo in his mind as well. The voice sounded strangely familiar but again he wasn't sure where he heard it before.
"Michikatsu... Michikatsu! Come on. It's time to go stargaze." The voice said the same phrase on repeat, yet it didn't drive Kokushibou crazy. He has heard it since his early years as a demon and it's the one thing that remained constant aside from stargazing.
When the full moon appeared, Kokushibou took it as his cue to leave. As he stood up the crunching of leaves came up behind him. He turned to see the source of the footsteps and caught a young woman with (h/l), (h/c) hair, (e/c) eyes and a strange red mark shaped like stars on her forehead. He immediately knew from her uniform and the sword strapped to her side that she was a demon slayer and judging by the strength in her aura she was most likely at the top of the top.
"A Hashira, eh." Kokushibou mumbled as he readied his sword, but the woman didn't draw her own even though she was standing in front of him albeit several feet away. Instead, she smiled widely. Pure joy gleamed through her eyes like a kid in a candy shop.
"Michikatsu?! Is that you?!" The woman asked followed by a laugh. Her voice was the same as the one that echoes in Kokushibou's head. He was taken aback by the woman to the point of dropping his sword while thousands of thoughts surged through his mind. "Oh, who am I kidding?! I know its you, Michikatsu!" The woman ran up to Kokushibou with her arms stretch out to hug him.
Instead of picking up his sword to finish her, Kokushibou placed his hand in front of him to keep her from hugging him. "I'm Kokushibou!" He said a little irritated.
"Kokushibou?" The woman titled her head. "Kokushibou? Sounds nice but you're Michikatsu to me!" The woman wriggled around and hugged Kokushibou. "I missed you so much. Remember when people used to call us the moon and stars and little brother Yoriichi was the sun."
The name "Yoriichi" rang a bitter tasting bell in Kokushibou's head but he couldn't remember a person by that name. "Who is that? Who are you?!"
(Y/n) laughed at his confusion. "I'm your sister (Y/n) and Yoriichi is our younger brother."
Sister? Brother? Must have been when I was human. Kokushibou surmised. But that was over 400 years ago. "Tsk. Whatever." Kokushibou picked up his sword and sheathed it before walking passed (Y/n) expect (Y/n) followed him.
"Hey, let's stargaze!" (Y/n) tried to pull him back to the cliffs but Kokushibou barreled passed her with his demon strength. (Y/n) quickly gave up and walked beside him. Kokushibou did nothing. Her presence actually felt welcoming in a sense that he wasn't alone. "So, where are we heading?"
"I'm going off on my own!" Kokushibou relied then took off running at top demon speed. Yet strangely when he stopped running, he felt... guilty for running away. He was also intrigued to know more about (Y/n). She had hinted that they were siblings on top of the fact that her presence and voice were very familiar. Kokushibou blinked a few times upon realizing that he was heading back in the direction of (Y/n) but didn't stop himself.
"Welcome back Michi, ni-san." (Y/n) was sitting down by the cliff almost like she was waiting for him. Kokushibou could see she was smirking but took it as joy. "Well, let's stargaze." Kokushibou sat down beside her without realizing it and looking up at the stars. After a long hour of silence, he spoke.
"So... you and this Yoriichi person are my siblings? I don't remember anything about my human life."
(Y/n) nods. "Yup!" Her smirk turned more sinister and she laughed slyly. "Why don't I help you!" She drew her sword lightning fast managing to gash Kokushibou's neck but not entirely behead him. He blinked once and she was gone. He immediately sensed her presence behind him and drew his sword.
"Moon Breathing Fifth Form: Moon Spirit Calamitous Eddy." Kokushibou slashed multiple times at (Y/n). Each slash was accompanied by crescent moon shapes. His eyes widened upon realizing that (Y/n) managed to evade them all unscathed. She's fast. I must run. He was running away before he knew it.
"Michi~! I'll see you again someday! HAHAHAHAHAHA!" (Y/n)'s sly laugh sent chills down Kokushibou's spine. The chills were replaced with rage knowing that (Y/n) could have easily killed him and that she was practically letting him run.
"Damn sister!" He shouted. "I'll kill you myself next time we meet!"
---
The battle between Kokushibou and Gyomei, Muichiro and Sanemi had persisted for a long while. The three Hashira were at the point of exhausted and though Kokushibou did fight himself on his toes occasionally, he had yet to falter.
"You've all fought well. Especially you," He points to Muichiro, "but such is to be expected from a descendant of mine. Your abilities all comes down to my cells being passed down. Now, farewell all of you." Kokushibou readied his blade. "Moon Breathing Tenth Form Drilling Slashes-Moon Through Bamboo Leaves." A triple layered slash twister whirled towards the exhausted Hashira.
"Hahahahahahahaha." The familiar sly laugh came. "Stars Breathing Second Form Constellation Orion." A constellation of stars shielded the Hashira from Kokushibou's attack.
(Y/n). Kokushibou frantically looked around for (Y/n) but couldn't find her. Then he caught sight of her diving down at him.
"Stars Breathing First Form Shooting Star's Descent." A cloud of stars emitted off her blade. Before Kokushibou could react, she had beheaded him as she swiftly flew passed him. "Now you can remember your past dear brother. I hope you understand, it's the only way to liberate your demon self." (Y/n) explained with a smirk on her face as she watched Kokushibou crumble to dust.
Kokushibou's memories came surging back in that moment. (Y/n) was right when she said that the two along with their younger brother Yoriichi were referred to the sun, moon, and stars. Yoriichi was the sun. He may have been quiet, but he radiated a warmth like that of the sun. His natural strength was one everyone could bask in except for Kokushibou who was known as Michikatsu in those days. He was moon. He couldn't surpass Yoriichi's own strength. No one noticed Kokushibou. It was much like how people sleep through the night when the moon shines. However, even the moon has the stars to keep it company.
(Y/n) was the stars to Kokushibou's moon. They were never apart even at a young age. (Y/n) would follow him into his career as a samurai and eventually into demon slaying when Kokushibou joined in hopes to surpass Yoriichi. One thing (Y/n) and Kokushibou did was stargaze before going on missions until one specific day.
"Michikatsu... Michikatsu! Come on. It's time to go stargazing." (Y/n) said. Michikatsu ignored her to continue training. Early that day, a group of demon slayers including Yoriichi had been assembled to confront Muzan but neither (Y/n) nor Michikatsu had been chosen. Michikatsu had been furious since that meeting thus leading him to train the rest of the day. (Y/n) pulled Michikatsu away from the training grounds. "Come on Michi. Let's go."
"No! I must train harder if I'm ever to surpass Yoriichi!" He pulled his hand out. He glanced back and patted (Y/n)'s head. "We'll go tomorrow. We'll stargaze a little longer to make up for it. I promise."
(Y/n) agreed to it. However, later that night...
"Michikatsu!" (Y/n) shouted to her brother. In front of him stood a man in formal attire with plum red eyes. "Michikatsu fight! That's Mu-" Before (Y/n) could warn Michikatsu about the man's identity she was swiftly decapitated and Michikatsu was turned into a demon under the guise of becoming more powerful than Yoriichi.
Kokushibou's crumpling head began tearing up much to (Y/n)'s surprise. "I'm sorry." Kokushibou said. "I was so overcome with jealously I didn't realize you were always there by my side till the very end." His mouth crumbled but his eyes took over to say what he needed to say next. "I'm sorry for not going stargazing with you."
(Y/n) nods silently but her eyes gleamed with forgiveness. It was the last thing Kokushibou saw before his head finishes crumbling.