Daki and her brother Gyutaro were one of the 12 Kizuki in Demon Slayer. This brother-sister duo is the only pair on the list to hold a position on Muzan's 12 Upper Moon tables in the series. All the other positions were held by individual demons, while Daki and Gyutaro were allowed to hold a position together.
For some reason, Muzan took a liking to this pair. He knew that Gyutaro would be an asset to him and thus chose to keep Daki as well in order to keep her brother in check. But it wasn't the demon lord who turned these two kids into demons.
Gyutaro and Daki as demons in Demon Slayer
Unlike many other demons, Muzan wasn't directly involved in turning these siblings into demons. Instead, it was Doma, the Upper Moon Two, who turned these two into flesh-eating monsters. During their lifetime, Gyutaro and Daki ruled the streets of the Entertainment District in Demon Slayer.
Growing up, both Daki and Gyutaro were neglected. Being born into poverty, this duo had very little going for them. Moreover, Daki was initially called Ume and was named after the disease that killed their mother. Gyutaro, on the other hand, was regarded as an ugly individual. Both of them had their own struggles that they slowly overcame.
However, as time progressed, Daki and Gyutaro grew strong. Daki was quite beautiful as an individual and was on her way to becoming one of the most popular oirans in the Entertainment District. Gyutaro, on the other hand, used his strength to his advantage and turned into a debt collector in the district.
Things changed for the worse when, one day, Daki stabbed one of her customers in the eye for not paying her. This individual turned out to be a Shinobi and attacked Daki, burning her alive. Gyutaro, on reaching the scene, found nothing but the burnt body of his sister. Although she wasn't entirely dead yet, he could do nothing to save her.
Having killed the man who hurt his sister so much, Gyutaro broke down, cursing the gods for snatching everything from him. To make things worse, no other human being had come to their aid. This caused the boy to hate all humans. It was at this moment that the then Upper Rank Six demon, Doma, showed up and shared his blood with these two individuals, resurrecting them as demons in Demon Slayer.
From then on, these two individuals started ruling over the Entertainment District and soon made their way into the ranks of the Twelve Kizuki. Their actions went unnoticed because the Entertainment District had a dark side as well that no one paid attention to in Demon Slayer. Their reign was finally stopped when the Sound Hashira Tengen Uzui, along with Tanjiro, Inosuke, and Zenitsu, made their way into the Entertainment District.
After a long-drawn fight between the demons and the Demon Slayer Corps, Daki and Gyutaro were finally killed, concluding the Entertainment District arc in the series.