He was so afraid of death that he allowed himself to become a Demon and therefore immortal. This makes him one of the few known Demon Slayers to defect and turn to become a Demon. In doing so he abandoned a wife and two children, though unlike many new demons he did not slaughter the vestiges of his human family.
Kokushibo was loyal to Muzan, acting as enforcer to the Twelve Kizuki. In return, Muzan sees him as a business partner, possibly due to his fanatic loyalty to him, being his strongest Demon and because the two were jointly responsible for almost exterminating Sun Breathing.
Kokushibo's character was quite contradictory. Even with the amount of times he felt jealous of his brother, or how he stated he hated his brother. He still held on to the precious flute that he gave to Yoriichi, because that was the only thing left that truly mattered to him.
Eventually, Kokushibo was killed by the duo: Sanemi and Gyomei. Even if Gyomei had gone solo against Kokushibo he could defeat the latter by awakening his Demon Slayer mark.
Even 400 years later, Muzan's fear of Yoriichi remained fully intact, as simply laying eyes on Tanjiro's hanafuda earrings caused him to recall their clash.
It is revealed that Muzan's true fear of dying and immense desire to live and survive was due to him being born "dead," where his heart would constantly stop in his mother's womb and was assumed stillborn until he fought to breathe before his cremation.
Rimuru Tempest becomes a God towards the end of the series and can travel back in time and travel between worlds. His space-time abilities alone would be enough to beat Yoriichi. This character is often stated to be stronger than Goku, despite the community being quite divided on this topic.
Tanjirō's resemblance to Yoriichi (like from Muzan's perspective), would trigger his memories and cause him to accept death. AND since its against Kokushibō and not Muzan, there's not poison that'll mess him up. So I would say Tanjiro would win.
While he reflects on his motivations, Gyomei is able to crush his head with his flail, again beheading Kokushibo.
4) Kokushibo
The Upper Moon 1 is a swordsman and is capable of causing a ton of destruction from any range. Nezuko will have to give her all to just keep up with someone like him. Kokushibo would easily be able to defeat her in a 1v1 combat.
In spite of his cruelty, Kokushibo is also an honorable person who gives respect to opponents he considered worthy.
He was soon decapitated by the combined force of Kanao and Inosuke, avenging Shinobu and her family. Trying to struggle for survival like Akaza, but lacking indomitable willpower as Akaza had, Doma's head slowly disintegrated and he met Shinobu's soul in the limbo, holding his severed head.
Yoriichi was powerful enough to defeat Muzan; however, he couldn't do so. That's because he couldn't come down to kill his twin brother, leading to his own demise. Even to this day, Muzan is scared of Yoriichi.
Trivia. Anne was originally planned to be Muzan`s older sister, who died by the hand of a Demon Slayer but that idea was scrapped, as the author knew practically nothing about Muzan`s early life. Instead, Anne became his wife, by which he had two children.
Haruka Shiraishi: Muzan's Daughter.
no. Yoriichi is not Tanjiro's dad, that would be Tanjuro. And Yoriichi is not related to the Kamado clan at all.
The last member, Upper Moon Four Nakime, ends up being killed by Muzan Kibutsuji himself to stop the Dimensional Infinity Fortress from being controlled by his enemies.
Unlike all other marks users in his era, Yoriichi was able to defy fate and lived to about 85 years of age (the average life expectancy during the era Yoriichi was presumed to have been born was between 30 ~ 50 years).
The Mist Hashira, along with Gyomei, Sanemi, and Genya, fought the Upper Moon 1. While the group was able to defeat them, both Muichiro and Genya died due to the wounds inflicted by Kokushibo.
1 Tanjiro's Patience Paves The Way To A Close Friendship With Inosuke Hashibira.
Kaigaku realizes that Zenitsu is much more versatile and skilled with his swordsmanship than before. However, he still believes that no matter how much Zenitsu trains, he can never defeat an Upper Rank like him.
Shinobu Kocho is a particularly concrete case for a Hashira Tanjrio could beat in Demon Slayer. As seen during the Infinity Castle arc, Shinobu does indeed lack the upper body strength to cut the head off a demon. This is why her Insect Breathing Style relies on quick jabs and Wisteria poison to kill her enemies with.
Yoriichi would be able to defeat every single Hashira on this list without breaking a sweat. Such was his reputation and skill in swordsmanship.
Yoriichi Tsugikuni, the first swordsmen to ever use Breathing Styles, once nearly killed Muzan with Sun-Breathing, and Muzan has been both physically and psychologically scarred ever since.
As the battle reached its climax, Tanjiro was able to knock the doll's head off and discovered that the mechanical body contained a mysterious sword. Fans of the manga will recognize this sword as the one carried by Yoriichi Tsugikuni himself.