Miyu Kuonji, the second eldest of three sisters, possesses a strikingly youthful appearance despite her twenties. She has long blond pigtails, blue eyes, and favors a short white dress with red buttons, paired with black knee-high socks and brown shoes. Her childlike stature frequently leads to mistaken identity, much to her vocal protest, though she occasionally accepts such treatment when incentivized by candy or games.

She boasts exceptional intellect, claiming an IQ of 240 and having graduated college early. Her patented inventions generate significant wealth, affording her ample leisure time. Among her creations is De Niro, an egg-shaped autonomous robot equipped with artificial intelligence, drills, rockets, and a voice capable of causing neurological harm. De Niro acts as her attendant and displays a bullying attitude towards electronic appliances.

Miyu exhibits a pragmatic and often blunt personality. She holds a documented attraction to younger boys (shotacon) and shows interest in Ren Uesugi, a servant in the Kuonji household. She endures frequent teasing and physical harassment from her eldest sister, Shinra, protesting loudly yet sometimes tolerating it for minor rewards. Ren first encountered Miyu when she collapsed from heat exhaustion, prompting him to assist her.

Within the mansion, Miyu employs Mihato Uesugi as her personal maid. She maintains a relationship with the youngest sister, Yume, though narrative focus remains strongest on her interactions with Shinra and Ren.