Zero is the elder sister of the powerful daiyokai Kirinmaru and paternal aunt to Rion. Six hundred years ago in the Heian period, she watched helplessly as Kirinmaru slaughtered a kingdom after samurai, under the Emperor's orders, attempted to kill him. During this event, Rion hid behind Zero and was fatally stabbed by the half-demon Sakasa, who controlled his headless body after decapitation. This incident fueled Kirinmaru's hatred of half-demons.

Zero harbored unrequited romantic feelings for Toga, the Great Dog Demon and rival to Kirinmaru. After Toga died protecting his human wife Izayoi and their newborn son Inuyasha, a tear Zero shed upon the Shikon Jewel carried a wish to cast away her emotions and demonic powers. This wish created seven Rainbow Pearls that sealed away her emotions and powers, scattering across the land to prevent their reunification. The Shikon Jewel had prophesied Kirinmaru would be felled by beings "neither human nor demon" and an "impossible entity surpassing time." Suspecting Inuyasha's quarter-demon daughter Moroha and Sesshomaru's twin half-demon daughters Towa and Setsuna could fulfill this, Zero targeted them.

To manipulate Sesshomaru, Zero forged a life-force link with Rin using the Silver-Scale Curse, ensuring Rin would die if Zero perished. Four years after the twins' birth, Zero discovered them in the Forest of the Sacred Tree of Ages and commanded the fire god Homura to burn it down, aiming to eradicate them. This act separated Towa and Setsuna, sending Towa to the modern era. Zero later stole sealing abilities from the monk Miroku to further her schemes, including unsealing Setsuna's demon blood to turn her against her family.

After Kirinmaru scattered the Rainbow Pearls to thwart her plans, Zero severed ties with him, declaring them strangers. She reclaimed several pearls, absorbing them back into her eyes. During this process, she inadvertently absorbed memories linked to the half-demon princesses, revealing Sesshomaru had anticipated her actions by placing the Gold and Silver Rainbow Pearls in his daughters' eyes at birth. Zero then allied with the demon Nanahoshi, disguising herself as "Otsuyu" to trap Towa on the night of the new moon. Using Nanahoshi's illusions and the Rainbow Pearls, she forced Towa to relive traumatic memories of isolation in the Reiwa era and Setsuna's forgotten bond, triggering Towa's transformation into a full demon, which Zero intended to exploit to eliminate her.

When Setsuna intervened and regained her memories of Towa, the sisters' reunion caused Towa to express gratitude to Zero for enabling her life in the modern era. This defiance, coupled with the spiritual apparition of Toga acknowledging Zero's unintended role in the twins' destiny, enraged Zero. She attempted suicide via her staff to kill both herself and Rin. Setsuna severed the thread of fate connecting Zero to Toga, destroying the Rainbow Pearls, restoring Zero's emotions, and freeing her from centuries of bitterness. Overwhelmed by remorse, she acknowledged her role in Toga's death and released Rin from the Silver-Scale Curse. She then revealed Rion's death to Towa and Setsuna, explaining Kirinmaru's vendetta against half-demons. Finally, she instructed Riku to kill her, dying peacefully after urging Kirinmaru to accept that the half-demon princesses might save rather than destroy him. Her body disintegrated, leaving only her staff behind.

Zero is depicted as a tall, slim woman with white hair pinned with a golden butterfly clip and two gold pins. She possesses green eyes similar to Kirinmaru and wears a black kimono tied with a large grey checkered-pattern ribbon, layered over a dark blue kimono and white robe. Her sleeves feature butterfly decals, and she wears black-painted nails with white tabi socks. Her personality manifests arrogance, vengefulness, and a willingness to employ underhanded tactics, contrasting with Kirinmaru's sense of honor. She harbors deep shame over the Rainbow Pearls, viewing them as symbols of her emotional vulnerability.