Ouran Nakagawa, called Ontan by friends, has been Kadode Koyama's best friend since they met at age eight. Her father operates Nakagawa Motors, her mother is politically active, and her older brother Hiroshi Nakagawa lives as a NEET earning money through affiliate marketing. Due to the family car business, she secured her driver's license in high school.

Initially shy and reserved in childhood, befriending Kadode transformed Ouran into an expressive, eccentric, and confident individual. Her present demeanor features erratic speech laced with internet slang and declarations about enslaving humanity or causing societal collapse, usually framed as exaggerated jokes or coping mechanisms. She drools when lost in thought, revealing her true self solely to close friends and family. Her primary interest is video games, especially first-person shooters, played nightly with Kadode.

A secret underpins her existence: she originates from an alternate dimension where Kadode, despairing after becoming an alien-tech vigilante, committed suicide. Grief-stricken, Ouran convinced an alien Investigator to shift her consciousness into this timeline to prevent Kadode's despair. Unintentionally, this act drew the alien mothership to Tokyo. Ouran retains no conscious memory of her dimensional origin but experiences related déjà vu.

Her bond with Kadode remains absolute throughout the story. She befriends an alien inhabiting a human corpse, "Oba," who reveals her dimensional displacement. During the crisis of the mothership's destabilizing reactor threatening global annihilation, she shares a kiss with Kadode. Her timeline manipulation triggers chain reactions: Kadode's father, Nobuo, later shifts to a new timeline where the invasion never occurred, enabling a peaceful life for both girls. This outcome indirectly fulfills Ouran's original goal of preserving Kadode's well-being.