The character known as Mushibugyō, also called Kuroageha or Black Butterfly, is the head of the Shin-Nakamachi Magistrate's Office, the institution commonly referred to as the Insect Magistrate's Office. She holds the high-ranking court position of Junior Second Rank and serves as a special elder within the shogunate's administration. Her primary responsibility is to oversee the organization tasked with protecting the city of Edo from the giant insects known as Mushi, which have been terrorizing the land for a century. She is one of the heroines of the story and plays a crucial role in the central narrative, with her true nature and origins being deeply tied to the overarching conflict.

In terms of appearance, Mushibugyō is a young woman with a slender, delicate build and long, beautiful silver hair. She stands at about 140 centimeters tall, giving her a youthful and almost fragile presence, and is often described as a pure and ethereal maiden. Despite her small stature and gentle looks, she carries an air of quiet authority and dignity befitting her station. She typically uses the refined first-person pronoun "warawa" in her speech.

Her personality is marked by a calm, reserved demeanor and a strong sense of duty toward her office and the people of Edo. She is not overtly expressive, but she shows warmth and care for those under her protection, especially the members of her magistrate's office. Her motivations are rooted in her commitment to defending the city and fulfilling the responsibilities entrusted to her by the shogunate. As the story unfolds, her personal history and the deeper reasons behind her involvement with the Mushi come to light, adding layers to her character and driving the plot forward.

Mushibugyō possesses a unique and dangerous ability: her body can produce a potent venom that is lethal to any living being that touches her skin. This makes her both a powerful asset in combat and someone who must be cautious in physical contact with others. When she unleashes the full extent of her power, she manifests large, dark butterfly wings, which is the origin of her nickname Black Butterfly. This transformation is visually striking and signifies the activation of her strongest abilities. Her body also holds a connection to ancient forces tied to the insect world, and this link becomes increasingly important as the story progresses.

Within the story, Mushibugyō initially appears as a mysterious and distant figure. The protagonist, Jinbei Tsukishima, does not immediately recognize her as his superior and refers to her simply as "the attendant" for a considerable period. Over time, their relationship deepens, and she becomes a key figure in his personal growth and in the team's efforts. She interacts with the other members of the Insect Magistrate, including the skilled swordsman Mugai, the kunoichi Hibachi, the convicted swordsman Koikawa Shungiku, and the young onmyoji Ichinotani Tenma, though she remains above them in the chain of command. Her bond with Jinbei evolves from one of distant authority to mutual trust and respect, and she eventually becomes one of his closest companions.

Throughout the series, Mushibugyō undergoes notable development. Her past, her connection to the Mushi, and the nature of her powers are gradually revealed, making her integral to the resolution of the main conflict. She is not merely a leader but a character with her own struggles and growth, and her abilities continue to be explored as she confronts increasingly powerful enemies. Her loyalty to her office and her willingness to protect those she cares about are central to her role, and her strength, both in terms of her venom and her transformation, makes her a formidable force in battle.