Natsu Iwashimizu is a 16-year-old girl and the main protagonist of Team Summer B in the series. She is physically small with childish features, short black hair, and light brown eyes. Her name contains the Japanese word for summer. Before the project, Natsu was a painfully shy and withdrawn high school student who was bullied and frequently refused to attend school due to her introverted and insecure nature. She lacked confidence and rarely spoke her mind, making her a social misfit. Her core motivation is survival in a radically changed world, and over time she also seeks to overcome her own fears and become someone who can contribute to her group.

In the story, Natsu awakens from cryonic preservation on a sinking boat alongside five strangers, thrust into a dangerous post-apocalyptic Japan. Her role within Summer B is initially that of a hesitant and seemingly unskilled member, but she gradually proves useful through her meticulous observation and documentation of edible plants, mushrooms, and other natural resources. She keeps detailed notes that assist the team in identifying safe food sources and understanding their environment. Her ability to name new organisms and her keen attention to detail become valuable assets.

Natsu’s key relationships include Arashi and Semimaru, who become her close friends and protectors. Arashi in particular acts as a steadfast support, helping her find her footing. Through these bonds and the constant trials of survival, Natsu undergoes significant personal growth. She learns practical skills such as basket weaving, begins to advocate for herself and others, and develops a greater sense of agency. Once indecisive and passive, she gradually becomes more active and resilient, embodying a growth-type protagonist who gains strength both internally and externally over the course of her journey.