Hailing from Planet Yuri, Naoko-san arrived on Earth under ambiguous circumstances, replacing Misuzu's elder sister. Her primary mission involves "yurifying" Earth into a lesbian-centric society through unconventional methods. She deploys technology to alter environmental elements, modifying trees to emit yuri-themed pollen for Misuzu's hay fever—though these trees subsequently launch into space like rockets.

Her alien physiology manifests in distinct traits. A communication device requires proximity to the lower half of a young girl's body to function. She summons trains to incapacitate threats like alien gropers using cartoonish, non-lethal violence. Her interests include reading yuri literature and erotic material, often leaving pornographic magazines scattered throughout the household. She visualizes scenarios of young girls playing in water during seasonal activities, drawn to their perceived "cuteness."

Relationships shape her interactions. As Misuzu's substitute sister figure, she occasionally assumes familial roles like replacing Misuzu's absent father, yet causes distress—such as falsely blaming Misuzu for scattered pornography. She commands Ryota, Misuzu's younger brother, subjecting him to strange tasks like posing in an adult store via robotic suit or enduring genital threats, while doubting his future romantic prospects. She clashes with Dark Naoko-san, a robotic duplicate on the family roof resembling Misuzu's original sister, intervening to separate Misuzu from her comfort—particularly during gestures like lap pillows.

Additional actions include manipulating technology to broadcast "sexier" television signals, inadvertently triggering widespread communal viewings. She facilitates shared fantasy experiences leading to prolonged communal sleep throughout summer. She cancels contracts for hired young girls at festivals but ensures their payment.