Kōsetsu Mie is an Iga ninja and the childhood friend of Ittoki Sakuraba. From a young age, she has been assigned to watch over Ittoki from the shadows, constantly following and observing him to ensure his safety. This duty is taken with complete seriousness, often disregarding his personal privacy. She is easily recognized by the black mask she always wears.

Her personality is quiet and reserved; she rarely speaks and tends to remain silent in most situations. Despite her taciturn nature, she demonstrates a genuine, if understated, care for Ittoki, her actions driven by a strong sense of loyalty to both him and the Iga clan. Her primary motivation is to protect Ittoki as the legitimate heir of Iga, and she fulfills this role with unwavering dedication.

In the story, Kōsetsu serves as a guardian and a constant presence at Ittoki’s side. After Ittoki discovers his ninja heritage, she accompanies him to the Kokuten Ninja Academy, enrolling as a student to continue her protective duties. There, she continues to monitor him and provides quiet, perceptive warnings about potential dangers, including the possibility of traitors among their newfound allies.

Her key relationships revolve around the Iga clan. She has a long-standing connection with Ittoki, rooted in their shared childhood, and works closely with his uncle, Tokisada Kaga, and his mother, Yumika Sakuraba, in protecting him. At the academy, she interacts with other students but remains focused on her mission. Over the course of the series, her dedication does not waver, though she becomes more integrated into the school environment and shows an increased concern for Ittoki’s well-being beyond mere duty.

Kōsetsu’s notable abilities align with her role as a ninja: she is highly skilled in stealth, surveillance, and close-quarters combat. Her constant vigilance and ability to act silently from the shadows make her an effective protector, and her commitment ensures that Ittoki is never far from her watchful eye.