Fifth-grader Natsuki Koyama, a Leo born August 21 with blood type O, recently moved from the city to the countryside with her mother Shimizu, now living alongside her cousin Amane Suzuki's family. Bursting with tomboyish energy, Natsuki fiercely aspires to be a heroine, idolizing heroic figures and positioning herself as her group's protector. She carries a cherished wasabi root in her bag during adventures – later revealed as a relative of the alien substance Zughan and coveted by antagonists for its power. Her habit of petting animals from the lower back upwards consistently unsettles them, sparking comedic chases.

Initial friction with younger cousin Amane stems from Amane viewing Natsuki as arrogant and lazy since the move. Tensions peak when their class pet rabbit, Pyon-Kichi, escapes under circumstances Amane blames on Natsuki. Confronting the reality of heroism, Natsuki faces fear and helplessness when Amane is abducted by Neppo's forces, forcing her to grapple with responsibility beyond simple fantasies. This catalyzes her growth towards a more complex understanding of courage and protection.

Throughout their space journey, Natsuki demonstrates decisive leadership and vital initiative in critical moments, her energetic approach proving crucial in navigating alien challenges to return home. Her character arc culminates in reconciling with Amane, their bond solidified through shared adversity and mutual support.