WARNING: The following contains major spoilers for the Demon Slayer: Kimetsu no Yaiba manga, by Koyoharu Gotouge, available in English through Viz Media.
The Upper-Ranks of the Twelve Kizuki are the most powerful demons in all of Demon Slayer: Kimetsu no Yaiba. However, Upper-Rank One stands head and shoulders above the rest, his power in a class all its own. In the manga's penultimate storyline, the "Infinity Castle" arc, it took the combined strength of Gyomei Himejima, Sanemi Shinazugawa, Muichiro Tokito and Genya Shinazugawa to defeat Kokushibo in a fight that saw both Muichiro and Genya lose their lives to the nearly unstoppable demon. So just who is Kokushibo, Muzan's most powerful demon and the only one whom the Progenitor of Demons seems to have actual respect for?
Kokushibo never once moved from his position as Upper-Rank One, and with Muzan's permission, he would consume any demon who challenged him to a Blood Combat to replace him. Akaza -- who challenged Kokushibo three times -- was the only one whom Kokushibo spared after defeating him as a sign of respect for Upper Three as a fellow warrior. Muichiro Tokito previously defeated Upper-Rank Five singlehandedly, but Kokushibo made him literally single-handed and ran him through with his own blade before Muichiro knew what happened. Even Himejima, whom Kokushibo called the most powerful Hashira in 300 years, was all but defenseless against Kokushibo once the Moon-Breathing demon got serious.
Kokushibo can use Breathing Techniques because, just like every other demon Muzan has ever created, he was once a human. However, unlike most demons, he was also a Demon Slayer. When he was a human, his name was Michikatsu Tsugikuni -- the twin brother of Yoriichi Tsugikuni, the first Sun-Breather, and creator of Breathing Styles. Unlike Yoriichi, Michikatsu was not a child prodigy as a swordsman. The only thing Michikatsu ever wanted was to be the greatest samurai in all of Japan, and the fact that his brother was not only a prodigious talent but had no interest in swordsmanship drove Michikatsu mad with envy.
Once Michikatsu and Yoriichi went their separate ways as children, Michikatsu eventually settled into a content life as a husband and father. However, when Yoriichi suddenly appeared one day as an adult and saved Michikatsu from a demon, he was reminded of his childhood dream and abandoned his wife and children to become a Demon Slayer. When Yoriichi joined the Demon Slayer Corps, none of them used or even knew about Breathing Styles, so Yoriichi taught them. Unfortunately, none of them, including Michikatsu, could master Sun-Breathing. Michikatsu came the closest with his Moon-Breathing, but his extraordinary skill was still nothing compared to his brother's.
Michikatsu even manifested his Mark, thus increasing his strength and skills even further, but it was not the Mark of a chosen Sun-Breather. In desperation to become a better swordsman before his Mark prematurely stole his life, Michikatsu accepted Muzan Kibutsuji's offer to become the first user of Breathing Styles to become a demon. After Muzan's near-death experience at the hands of Yoriichi, he and Michikatsu -- now officially known as Kokushibo -- traveled across Japan eliminating anyone who knew of Sun-Breathing. While they somehow missed the Kamado family, they actively avoided Yoriichi, knowing that neither one of them stood a chance against him.
Those who manifest their Mark are meant to die by age 25, but Yoriichi lived to be 85 years old, and even as a decrepit old man, he nearly killed Kokushibo when the two met for the first time since Michikatsu became a demon. When Yoriichi died of old age on his feet in front of Kokushibo, the Moon-Breathing demon angrily obliterated how brother's body, shouting that he hated him for making him feel so shamefully inferior. In the "Infinity Castle" arc, as Kokushibo's body withered away from the wound that a dying Muichiro Tokito dealt him with a burning-red katana, he anguished over why Yoriichi always made him feel so awful. In his last moments, Kokushibo acknowledged that the only thing he ever wanted was to be as strong as Yoriichi.
