Natsuki Shinomiya attends Saotome Academy as a member of the idol group ST☆RISH. He stands 186 cm tall with blond hair and light green eyes, typically wearing oval-lens glasses. His appearance is consistent, though removing his glasses triggers a transformation into his alternate personality, Satsuki.

He displays a gentle, airheaded nature and an intense obsession with small, cute things. This leads him to frequently compare classmate Haruka Nanami to his dog Elizabeth due to their perceived resemblance. His obsession also extends to dressing his roommate, Syo Kurusu, in costumes and photographing him against Syo's will. Natsuki enjoys cooking but remains unaware of the disastrous effects of his creations, which incorporate unconventional ingredients like tabasco sauce, chili sauce, fermented soybeans, and fermented squid. These dishes routinely cause illness or unconsciousness in others, though he himself remains unaffected.

Musically, Natsuki possesses genius-level talent, particularly with string instruments. He initially played the violin competitively in childhood alongside Syo but later specialized in the viola. He perceives music as "stars transmitting pretty words into his head," enabling effortless composition. This talent connects directly to his psychological condition, Gemini syndrome, manifesting as a split personality. The removal of his glasses transforms him into Satsuki—a solitary, volatile persona possessing heightened physical strength and a short temper. Satsuki acts as Natsuki's protector and demonstrates exceptional compositional abilities during his control. Natsuki retains no memory of Satsuki's actions, which historically resulted in numerous hospitalizations. The condition originated in childhood after a music teacher plagiarized his composition "Satsuki," causing profound betrayal and emotional trauma.

His history includes living in France until age five, followed by a move to Hokkaido. His childhood friendship with Syo involved violin competitions. One incident at ages five (Syo) and seven (Natsuki) caused Syo's acrophobia: Natsuki chased Syo up a tower, leading to a near-fatal fall where Natsuki rescued him. This event resurfaced during hypnosis therapy in their academy years.

His development shows gradual reconciliation with his dual identity. Early depictions show no awareness of Satsuki, though he experiences vague sensations of being watched. After a rooftop confrontation where Satsuki emerges following Haruka's rejection of Natsuki's romantic confession, Natsuki actively seeks understanding of his condition. Through interactions with Haruka and Syo, he confronts his past trauma, including the teacher's betrayal. This culminates in acceptance of Satsuki as part of himself, leading to personality integration in later seasons. A pivotal moment occurs when he voluntarily removes his glasses without transforming, demonstrating newfound control.

Relationships significantly influence his growth. His bond with Syo blends camaraderie and conflict, particularly regarding Syo's fear of heights and resistance to being called "cute." With Haruka, he shares musical collaboration and emotional trust, confiding in her during vulnerable moments like revisiting his violin playing. He promises to show her the Southern Cross constellation, symbolizing their shared future. Academy principal Shining Saotome aids his journey by orchestrating interventions like hypnosis and disclosing his condition during a physical examination.

Natsuki's strength, inherited from his father, and his family's farming background are noted traits. Despite early struggles with identity and trauma, his narrative concludes with psychological wholeness and continued musical pursuits within ST☆RISH.