Natsuki Shinomiya is a tall young man with blond wavy hair featuring one distinctive upward-curling lock and light green eyes. He constantly wears oval-lens glasses crucial for more than vision correction. While his physical appearance remains unchanged without them, his demeanor shifts drastically due to the emergence of his alternate personality.

His primary personality is gentle, tender, and air-headed, harboring an extreme fondness for small and cute objects or people. This compels him to immediately liken Haruka Nanami to his dog Elizabeth and frequently attempt hugs on her or others he finds adorable. A musical genius, especially with string instruments, he demonstrates unexpected insight during significant moments despite his usual naivety. He enjoys cooking yet remains completely unaware that his creations—often incorporating bizarre ingredients like fermented squid or tabasco sauce—are inedible, causing severe physical reactions in those who consume them. His roommate Syo Kurusu frequently becomes an unwilling taste tester.

Removing his glasses triggers a shift to Satsuki, characterized by heightened aggression, solitary behavior, and immense physical strength. Satsuki possesses a volatile temper and has reportedly hospitalized 50,000 individuals through violent outbursts. Paradoxically, Satsuki is also a musical prodigy who composes intricate pieces during solitude, explaining Natsuki's belief that music "appears" to him spontaneously. Natsuki retains no memory of Satsuki's actions once glasses are restored, the only method to revert his personality. This "Gemini syndrome" originated from childhood trauma: as a violin prodigy, he composed "Satsuki" for an admired teacher, who plagiarized his work for fame. The resulting despair and anger fractured his psyche, creating Satsuki as a protective entity.

His history includes residing in France until age five, followed by a move to Hokkaido. He met Syo Kurusu in elementary school, where both competed in violin competitions. A pivotal incident occurred when seven-year-old Natsuki chased five-year-old Syo up a tower, causing Syo to fall from a window. Natsuki saved him but inadvertently instilled Syo's lifelong acrophobia. Neither recalled this event until their academy director, Shining Saotome, recovered the memory through hypnosis. After the plagiarism trauma, Natsuki abandoned the violin for the viola but later quit altogether, feeling unable to "find the sound he wanted." He enrolled in Saotome Academy seeking to rediscover his joy in music.

Throughout his academy years, recurring conflicts arise from accidental glasses removal. In Season 1, incidents include a pool match where a water polo ball dislodges his glasses, leading to Satsuki's rampage until Haruka restores them. During a cultural festival, he confesses feelings to Haruka on a rooftop; rejection causes his glasses to break, triggering Satsuki's violent emergence. This results in Syo moving out and Haruka seeking reconciliation. His development peaks in later seasons as he gradually reconciles with Satsuki, accepting this aspect of himself. This integration resolves his amnesia regarding Satsuki's actions and stabilizes his duality.

He shares a dormitory with Syo Kurusu. Syo's acrophobia and frequent evasion of Natsuki's affectionate gestures fuel comedic interactions. Despite this, they maintain a deep bond rooted in childhood camaraderie and musical rivalry. Natsuki also develops a protective relationship with Haruka, promising to show her the Southern Cross constellation after their debut and frequently involving her in his culinary experiments.

His theme color is yellow. He stands 186 cm tall, making him the tallest in his idol group. His family background is on a farm, and his strength is inherited from his father. He shares a birthday (June 9) with the Kurusu twins.