Shiina Arizuki is a young girl with light brown hair styled in short quadtails and dark blue eyes. She typically wears a dress decorated with rape blossom flowers and sunflower-shaped hair ornaments, stands 125 centimeters tall, and weighs 25 kilograms. Cheerful and energetic, she readily befriends strangers like Kudryavka Noumi and Riki Naoe during her search for a plastic bottle to win a water rocket competition.

She owns a Welsh Corgi named Ousumi, referencing Japan's first satellite. Shiina dreams of becoming an astronaut, a goal her family opposes. This ambition originates from her background: her father, an associate professor and engineer, helped develop the "Nanohana" (rape blossom) satellite, with the "na" in her name reflecting this connection. After her parents divorced during her childhood and her father moved to Germany, Shiina lived happily with her mother and older sister, Ui Arizuki, sharing moments like bathing together. When contact was lost with her father's satellite, Shiina vowed to retrieve it, sparking her interest in space. This passion led her to space-related toys and studies but strained her family relationships and caused classmates to view her as unusual.

Her older sister, Ui Arizuki, is a second-year high school student who stays at a school dormitory despite living nearby. Ui is a realist who doesn't believe in achieving dreams, works part-time at a family restaurant, and stopped playing lacrosse due to an injury. The sisters share the surname Arizuki, meaning "exist" and "moon". Shiina's given name combines "chinquapin, mallet, spine" and "vegetables, greens". Symbolically, her sunflower hair accessories reference Japan's Himawari satellites. Shiina's narrative focuses on her unresolved familial relationships and her steadfast commitment to becoming an astronaut despite opposition.