No.0 acts as an advisor to the Council of the Seven Virtues, recruiting Souta Hatate to oppose the humanity-threatening Angelus Gemeni. She later reveals herself as Kagura Bladefield, the founding queen of the Principality of Bladefield. This confirms her place within the Bladefield royal lineage, linking her to central figures like Nanami Knight Bladefield and Souta, identified as Nanami's younger brother.

Kagura possesses knee-length blonde hair, braided on the right side, and red eyes. Her name carries the meaning "Shinto music." She approaches Souta during a festival and recounts a tale of a boy sacrificing his life to save two princesses, becoming a flag. Souta recognizes echoes of his own traumatic past in this story, plunging him into despair before Kagura departs.

Later, Kagura confronts Souta directly, warning that his continued existence risks the world's destruction. She urges him to stop using his flag-manipulating abilities to alter fate. When Souta refuses and joins a dance, Kagura freezes time. She explains that parallel universes function as simulations designed to find a method for humanity to withstand the Angelus Gemeni. Her overarching objective is to utilize these realities to develop strategies for humanity's preservation.

Kagura's actions and disclosures propel Souta towards a climactic confrontation where he must reconcile his past and confront the Angelus Gemeni to protect his world. Her role intertwines with deeper Bladefield lore, including the manifestation of supernatural abilities in certain family members across generations and the truth behind fairy tales concerning the kingdom's origins.