Nagisa Takawashi is a petite first-year high school student, standing 152 cm tall and weighing 38 kg, with physical measurements of 76-53-78. She possesses blonde or yellow hair and brown or yellow eyes, with blood type A and the Gemini zodiac sign. She is Yuuichi Takawashi's younger sister.

Nagisa exhibits a strong romantic attachment to her brother, expressing affection more akin to a lover than a sibling. This creates tension as Yuuichi perceives these feelings as inappropriate. Her fixation manifests through frequent fantasies involving him, sometimes accompanied by acts of self-gratification.

Her interests and activities focus on domestic tasks, listing cooking and household chores as hobbies. Within the narrative, she is part of a complex relational dynamic involving six high school students navigating interpersonal conflicts and romantic entanglements during their final academic year. Her role specifically contributes to themes of complicated attachments and boundary-crossing desires.