Eimi Haneoka is Meimi Haneoka's mother and the wife of professional stage magician Genichiro Haneoka. A retired thief, Eimi left that life behind upon marrying Genichiro, becoming a housewife. Her past significantly shaped their daughter Meimi, influencing Meimi's skills and activities as the phantom thief Saint Tail. The ruby in Eimi's engagement ring inspired the name of Meimi's pet hedgehog, Ruby.

Eimi actively supports her daughter, sometimes covering for Meimi's nighttime excursions. While her full awareness of Meimi's secret identity as Saint Tail isn't explicitly confirmed across all media, Eimi demonstrates knowledge of the nocturnal activities, providing alibis and plausible explanations for Meimi's absences. This indirect assistance hints at a tacit understanding. Eimi expresses hope that Meimi might follow her own former path, a wish threaded with Genichiro's contrasting desire for Meimi to pursue stage magic.

Her character consistently functions as a maternal figure bridging Meimi's ordinary life and secret endeavors.