Muzan Kibutsuji, the manga's main antagonist, didn't get actively involved in the conflict with the Demon Slayer Corps until the manga's final arc. Prior to then, the six Upper-Ranks of the Twelve Kizuki acted as the main adversaries for the most of the narrative. The "Mugen Train" arc's battle with Lower-Rank One Enmu represented Tanjiro Kamado's final encounter with a foe other than the most powerful demons.
Yet, it should not be surprising that none of the Upper-Ranks survived to the end of the series given that they were merely Kibutsuji's goons. Let's examine what will happen to each Upper-Rank character by the manga's conclusion.
Upper-Rank 6: Gyutaro and Daki
Gyutaro and Daki battled Tanjiro, Tengen Uzui, Inosuke Hashibira, Zenitsu Agatsuma, and Nezuko Kamado during Demon Slayer's "Entertainment District" arc. Tengen, the Sound Hashira, was forced into early retirement after losing his left hand and left eye in the battle to beat them, despite the fact that Iguro Obanai referred to them as the "lowly" Upper-Six.
Tengen assisted Tanjiro in beheading Gyutaro, and Zenitsu and Inosuke worked together to behead Daki. Before leaving for Hell together, Gyutaro and Daki reaffirmed their love for one another as brothers and sisters. Like most demons, their impending death brought back memories of their former lives as humans.
Upper-Rank 5: Gyokko
In the "Swordsmith Village" storyline of Demon Slayer, Gyokko was one of the Upper-Rank demons that assaulted the titular settlement. Gyokko was the only Upper-Rank in the series to go 1v1 with a Hashira and lose against Muichiro Tokito's Mist Hashira. Gyokko was an artist whose pride in his horrifying works, which he made from the bodies of those he had killed, finally brought to his own death.
Gyokko, a fellow artist, was drawn away by Haganezuka's devotion to sword-polishing and decided to postpone killing a wounded Muichiro. Tokito then appeared with his Mark and used Mist-Breathing, Seventh Form: Obscuring Clouds to easily decapitate Gyokko. Unlike other demons, Gyokko didn't have time to lament his actions or regain his human memories, as Muichiro split Gyokko's decapitated head in half to get him to stop talking.
Upper-Rank 4: Hantengu
Together with Gyokko, Hantengu attacked the Swordsmith Village and engaged in combat with Mitsuri Kanroji, Tanjiro, Nezuko, Genya Shinazugawa, and Tanjiro. Hantengu was exceptional in that having his head severed only made him stronger. Over the course of the combat, he dispersed into seven distinct bodies, and Tanjiro and the others battled to locate the genuine body whose beheading would end his life.
The only occasion in Demon Slayer that Muzan joyfully complimented one of his demons was when Hantengu forced Nezuko into the sunshine only for her to escape exposure. Despite the fact that Hantengu was ultimately vanquished by Tanjiro. Hantengu, like Gyokko, didn't have much time to consider his human memories as most dying demons do because his body was exposed to the sun right after he was beheaded.
Upper-Rank 3: Akaza
Akaza didn't return to the action after his debut in the "Mugen Train" arc until Demon Slayer's "Infinity Castle" arc. Akaza defeated Tanjiro and Giyu Tomioka, demonstrating his unbeatability. Even after Tanjiro neutralized the Compass Needle and severed Akaza's head from his body, the head still grew back. Akaza's Compass Needle ability allowed him to anticipate his opponent's tactics. If being beheaded hadn't brought back memories of Akaza's previous life as a human, the worn-out Tanjiro and Giyu would have perished at Akaza's hands.
Hakuji, a teenage martial artist, lost his teacher and fiancée when a competing dojo contaminated their well. As Kibutsuji arrived to look into the demon-level havoc Hakuji had wreaked on the rival dojo, he turned into a demon. When Akaza thought of his deceased fiancée, he felt guilty about the monster he had become and destroyed his own recently revived head, so enabling himself to perish.
Upper-Rank 2: Doma
During the "Infinity Castle" arc, Doma was first confronted by the Insect Hashira, Shinobu Kocho, whom he quickly consumed after her poisons failed to kill him. Kanao Tsuyuri and Inosuke then teamed up to fight, but were no match for him until the 37 kilograms of Wisteria poison he'd inadvertently ingested by consuming Shinobu began to melt his body. Doma was described by Kanao (accurately) as completely empty inside.
Even as he was dying, Doma acknowledged that he didn't feel anger at losing or even fear of dying, but rather he felt the same thing he'd always felt even as a human -- nothing. After being decapitated by Kanao and Inosuke, Doma experienced his first emotion in his entire life when he met Shinobu in the afterlife and, despite her hatred for him, began to develop a crush on her.
Upper-Rank 1: Kokushibo
Kokushibo was the final Upper-Rank to die during Demon Slayer's "Infinity Castle" arc. Muichiro, Genya, Sanemi Shinazugawa and Gyomei Himejima fought together against Kokushibo, though Muichiro and Genya were no match for him and were killed. Sanemi and Gyomei decapitated Kokushibo, but he successfully regenerated his head just as Akaza had. However, the wound from Muichiro's burning red blade caused his body to disintegrate anyway.
As his body withered away, Kokushibo reflected on his human life as Michikatsu Tsugikuni, the twin brother of Yoriichi. Michikatsu always felt inferior to his brother because only Yoriichi could wield the all-powerful Sun-Breathing Technique. He chose to become a demon so he could live forever and improve his sword skills, though as he died, he admitted he'd just wanted to be as strong as Yoriichi.
