Natsumi Aizawa is a central character in Natsuiro Kiseki. She is a second-year middle school student and a member of her school's tennis club. Raised by a single mother, she is accustomed to helping with household chores and takes on a responsible role caring for her younger brother, Daiki. Natsumi is bright, energetic, and straightforward in nature, often acting before thinking things through, which can lead to impulsive decisions. She is deeply loyal to her friends, especially Saki Mizukoshi, who lives next door and has been her close friend and tennis doubles partner since childhood.

At the start of the story, Natsumi's stubbornness and frustration cause a serious falling out with Saki after Saki suddenly quits the tennis club and reveals she is moving away. This conflict drives much of the early tension as Natsumi struggles with feelings of hurt and betrayal. Her motivations are rooted in a desire to preserve the close bond the four friends share, and she gradually learns to accept change and communicate more openly. Throughout the series, Natsumi takes an active role in the group's adventures, often showing a protective side toward her friends and family. Her tennis skills are a notable ability, and in one episode, the magic of the rock creates a duplicate of her so she can manage both training and household duties, eventually combining the experience and strategy from both to improve her performance. Over the course of the summer, Natsumi matures emotionally, reconciling with Saki and learning to value the time they have left rather than resisting the inevitable. Her relationships with the other three girls are central to the story, with her dynamic with Saki being the most prominent, while her interactions with Rinko Tamaki and Yuka Hanaki highlight her role as a grounded yet passionate member of the group.