Daki, originally named Ume, entered the world in Yoshiwara's impoverished Rashomon Riverbank, a forsaken sector where destitute sex workers were discarded. Her mother, an unstable prostitute, neglected and abused Ume and her older brother Gyutaro before succumbing to disease during Ume's youth. Gyutaro raised his sister, observing how her extraordinary beauty attracted gifts and opportunities, earning her the moniker Shiraume. At twelve, a prestigious brothel recruited Ume, educating and training her to become an oiran. Her beauty generated substantial income while Gyutaro worked as a debt collector. This ended violently when Ume, defending her brother's honor, blinded a samurai client with a hairpin. In retaliation, she was bound, burned alive, and left for dead. Gyutaro discovered her near-fatally injured, killed the samurai and brothel manager, then carried Ume through Yoshiwara seeking aid. Exhausted and wounded, they encountered Upper Rank Six demon Doma, who transformed them into demons to save their lives.

As a demon, Ume adopted the name Daki, meaning "fallen princess," reflecting her new existence. She operated in Yoshiwara for over a century, masquerading as the oiran Warabihime to lure victims. Daki exhibited extreme pride, cruelty, and sadism, tormenting victims before killing them and bullying brothel workers into suicide without remorse. She selectively consumed only those she deemed beautiful or strong, discarding others to stage apparent suicides. Despite her age, Daki frequently displayed childish tantrums and hysterical crying when thwarted, traits linked to her transformation at age thirteen. Her personality was heavily shaped by Gyutaro's teachings of selfishness and preemptive aggression, though she remained fanatically loyal to Muzan Kibutsuji, obeying him with fearful reverence. She harbored profound pyrophobia stemming from her human-era burning, reacting with paralyzing terror to fire-based attacks like Nezuko Kamado's Blood Demon Art.

Daki shared the Upper Rank Six position with Gyutaro, though her individual power was lesser. Her abilities included manipulating sentient obi sashes generated from her flesh; these sashes could extend indefinitely, slice through structures, and function autonomously. They regenerated rapidly, and she could transform her neck into flexible material to resist decapitation. When dispersed across Yoshiwara, her strength diminished, but reclaiming them restored her full power, turning her hair silver-green and enhancing her speed and durability. She could fuse biologically with Gyutaro, enabling him to reside within her body and emerge when she was beheaded or overwhelmed. Gyutaro's eye implanted in her forehead granted shared senses, improved reflexes to counter techniques like Zenitsu's Thunderclap and Flash, and synchronized attacks. Daki also possessed standard demon traits like accelerated regeneration, agelessness, and sunlight vulnerability. Her neck-extending ability drew inspiration from the rokurokubi yokai legend.

During the Entertainment District conflict, Daki killed and consumed seven Hashira. She murdered Kyogoku House manager Omitsu for discovering her secret, dropping her from a height. Posing as Warabihime, she abused staff and suspected intruders like Zenitsu Agatsuma, whom she punched through walls. She attempted to consume the oiran Koinatsu but was intercepted by Tanjiro Kamado. After initial clashes, Daki regained her full power and bisected Nezuko Kamado; Nezuko regenerated and burned her, triggering Daki's pyrophobia. Gyutaro then emerged, implanting his eye to enhance her abilities. The siblings coordinated attacks against Tengen Uzui and the Demon Slayers until Zenitsu beheaded Daki simultaneously with Tanjiro and Tengen beheading Gyutaro.

In limbo, Daki rejected Gyutaro's attempt to send her to heaven alone, insisting they stay together. She tearfully reaffirmed their bond, recalling his promise never to leave her and vowing to remain his sister across rebirths. Gyutaro reflected that her human self was impressionable and honest, and that proper upbringing might have yielded a kinder person.