Natsuki Momohara is a fourteen-year-old middle school student spending her summer vacation at her older brother's Tokyo apartment. Her stated purpose is to provide him with life guidance and ensure his well-being while he is occupied with work. She has chest-length black hair, usually worn in low twin braids with bangs across her forehead, and possesses distinctive pink eyes.

Natsuki is naturally responsible and caring, particularly toward her brother, and is highly proficient in all matters of housework. Despite a youthful and naive appearance, she confidently takes charge of her brother's domestic life during her stay. She often masks her true feelings, however, preferring indirect expressions over straightforward communication.

Her days in Tokyo revolve around caring for her brother and diligently managing the household. These shared routines gradually rekindle warmth and a familial connection for him. Their dynamic shifts when her brother, stirred by impatience or guilt over her constant attention, speaks to her coldly. This provokes Natsuki to openly express her concealed emotions, leading to a mutual understanding before her summer vacation ends and she returns home.

Her given name, Natsuki, combines the characters for "apple tree" and "moon," while her surname, Momohara, means "peach field" or "peach plain."