Natsuki Momohara is a fourteen-year-old middle school student and the younger sister of the male protagonist in the anime One Room. She travels to Tokyo to stay with her brother during her summer vacation, living in his small apartment for the duration of her visit. Natsuki has long black hair and pink eyes, and she is depicted as an energetic, caring, and sometimes mischievous girl who enjoys taking care of her brother, though she often struggles to express her true feelings.

Her personality is marked by a blend of meddlesome affection and shyness. She frequently involves herself in her brother's daily life, offering to clean, cook, and help around the apartment, actions that reflect her desire to be close to him and to be helpful. However, Natsuki is also easily embarrassed and tends to deflect her feelings with teasing or playful behavior. This inner conflict is highlighted by her tendency to blush and become flustered when her brother pays her a compliment or when she finds herself in a situation that makes her emotions obvious. She is often unable to be straightforward about her affection, resorting to indirect actions or comedic excuses to mask her shyness.

Natsuki's motivation stems from a genuine wish to strengthen her bond with her brother and to create lasting memories during their limited time together. She looks up to him and wants to be seen as more than just a little sister, yet she is still learning how to navigate her own feelings. Her role in the story is one of the three primary heroines in the first season, with her narrative arc focusing on the small, intimate moments that develop between siblings as they share a living space. Key relationships center entirely on her brother, as the series is presented from a first-person perspective through his eyes. There are no other significant recurring characters in her storyline.

Over the course of her episodes, Natsuki shows subtle development. She begins by acting confidently and taking charge of household tasks, but gradually reveals her vulnerability and the difficulty she has being honest about her emotions. In later appearances, including the third season, she continues to mature, learning to express herself more openly, though her core personality of being a caring yet bashful younger sister remains consistent. Natsuki does not possess any supernatural or extraordinary abilities; her notable traits are her practical housekeeping skills, her knack for cooking, and a clever but transparent technique taught by a friend that she occasionally uses to try to get her brother's attention.