Natsumi Aizawa is one of the four main characters in the anime 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 good care of her younger brother, Daiki. Natsumi has short orange hair and red eyes, and she is described as a bright, energetic, and honest girl. She lives next door to Saki Mizukoshi, who has been her best friend since childhood, and the two often play as a pair in the tennis club.

Natsumi is passionate and sometimes impulsive, acting quickly without always thinking through the consequences. She deeply values her friendships, especially with Saki, Yuka Hanaki, and Rinko Tamaki, the other three members of their close-knit group. When Saki announces that she will be transferring schools, Natsumi becomes upset and initially falls out with her. This conflict drives much of the early story, but the girls eventually reconcile with the help of a magical rock at a local shrine that is said to grant wishes when four close friends wish for the same thing.

Throughout the series, Natsumi shows growth in her ability to support her friends and accept change. She is also musically inclined, playing the electric guitar and dreaming of becoming a famous musician. Her love for music is a shared interest that helps strengthen the bond among the four friends. In terms of abilities, Natsumi does not possess any supernatural powers herself, but as part of the group, she experiences the rock's magic, which manifests in various ways such as the ability to fly, body swaps, and duplication. Her role in the story is that of a loyal and spirited friend who helps drive the narrative forward, particularly through her strong emotional connection with Saki and her willingness to embrace the summer's miraculous events.