In the end, Kokushibo died due to Muichiro's Bright Red Nichirin Sword hampering his regeneration, and the realization of how ugly he has become in his pursuit of strength causing him to lose the will to fight.
The fight with Kokushibo, the highest-ranking demon among Muzan's servants, is over. Although Himejima and Sanemi defeated Kokushibo and sent him to hell, the price the Demon Slayer Corps has paid is very high.
After various battles within the Dimensional Infinity Fortress the remaining Upper Moons are killed by the hands of various Demon Slayers. 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.
Fortunately, Zenitsu was able to single handedly beat the Upper Moon 6. Not only did he defeat the demon, he also created a seventh form called the Flaming Thunder God. These are the demons who have been given the title of Upper Moon 6 in Demon Slayer.
Tanjiro decapitates Akaza with the Hinokami Kagura: Setting Sun Transformation. Akaza was bewildered, wondering how Tanjiro did not have any fighting spirit, comparing this feeling to fighting against a plant. He acknowledges that Tanjiro surpassed his speed.
We first see Doma when he is still the Upper Moon Six while he is in the middle of eating a geisha. Later he becomes even stronger and rises to the position of Upper Moon Two. He dies because of the lethal amount of poison he absorbed through Shinobu Kocho's body followed by Kanao Tsuyuri beheading him.
However, Doma remained unfazed by the deaths of his own parents. When he was twenty years old, Doma met Muzan Kibutsuji, who granted him power and turned him into a demon.
After becoming a Demon Daki grew strong enough to kill and eat seven Pillars and gained the attention of Muzan Kibutsuji who allowed them to join the Twelve Demon Moons. She later dies after being beheaded by Zenitsu Agatsuma and Inosuke Hashibira.
With that said, all Hashira in Demon Slayer have one glaring flaw: they're only human. As such, they're not infallible and are certainly not tougher than their demon enemies. This results in some rather lopsided matches and emotionally-charged oversights on their part.
As it turns out, Tanjiro has received a few promotions during his service. However, saying that Tanjiro has become a Kanoe doesn't mean much on its own. The Corps' ranks and functions are fairly straightforward.
At some point, Daki fought, killed and ate seven Hashira. Two days before Tanjiro Kamado and company infiltrated the Entertainment District, Daki was confronted by Omitsu, the manager of the Kyogoku House, over her mistreatment of the staff.
After the deaths of Daki and Gyutaro, Kaigaku was promoted to become Upper Moon Six of the Twelve Demon Moons due to his power and confronted Zenitsu as a rematch he always desired.
When it comes to pure strength, Shinobu Kocho is the weakest Hashira in Demon Slayer. She doesn't even have enough strength to decapitate a demon's head and she makes up for it by injecting poison into the demons.
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.
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.
The answer to whether Upper Moon One is stronger than Muzan in Demon Slayer is quite clear; Kokushibo is not stronger than Muzan Kibutsuji. The main piece of evidence here is that Muzan was able to escape a living Yoriichi, whereas Kokushibo needed death's assistance to prevail.
Demon Slayer's Tengen Uzui is considered to be one of the weakest Hashira due to almost losing this fight, but the fact is that Tengen only looks weak when compared to the most overpowered Hashira.
5/14 Fastest Hashira In Running
Despite his large and muscular build, Tengen Uzui is the fastest runner among the Hashira.
Nezuko setting Tengen on fire. By the end of the battle, Gyutaro's poison has spread throughout Tengen's body resulting in his inability to speak. As he lays dying from the poison, Nezuko Kamado intervenes and uses her her Blood Demon Art to burn the poison out of his system, allowing him to rejoice with his wives.
Tengen lost his arm and was seen lying on the ground unconsciously because of the effect of Gyutaro's poison. Subsequently, he rejoined the battle by protecting Tanjiro with one blade in his right hand and the other clenched in his mouth.
The only two people she really cares for are Muzan Kibutsuji and her older brother. She shows deep respect for Muzan Kibutsuji immediately falling into a bow when he appears. Although she cares and loves her brother she is seen insulting and blaming him at times when things aren't going well.
Because of his desire to become stronger and Doma's strength allowing him to surpass himself to become Upper Two, Akaza harboured an intense hatred of Doma. Doma, as apathetic as he is, considered him his "best friend", seemingly ignoring the contempt Akaza possessed.
As a Human
Over a century ago, Doma was born a child with extraordinary features. His rainbow-colored eyes and pale hair led people to consider him special and believed he could communicate with the gods.
Shinobu musters strength for one final attack, successfully stabbing Doma through his neck with a poison strike. Shinobu triumphantly relaxes as Doma seemingly begins to die from the poison. However, Doma almost instantly recovers and bear hugs the Hashira, absorbing Shinobu into himself and eventually killing her.