Kaitō Reinya is the protagonist of her namesake series, a phantom thief who leads a double life as a seemingly ordinary store clerk. During the day, she works at a small shop called Faminiya under the name Reina, presenting an unassuming face to the world. When night falls, she adopts her alter ego as a magical girl-themed thief, carrying out elaborate heists with the help of her small yellow mouse companion, Chūtarō, whom she also uses as an advisor for her plans.

Reinya's personality is defined by the sharp contrast between her public persona and her true nature. As a magical girl, she embodies a whimsical and fantastical appearance, but her real character is pragmatic, cunning, and resourceful, possessing a sharp sense of humor and a talent for improvisation. Her speech is noted for the use of the Hakata-ben dialect. While she is a criminal, she is not portrayed as a villain; her actions are framed with a comedic tone, and her targets are often corrupt or morally dubious individuals, resulting in her technically illegal acts frequently leading to positive outcomes. Her motivations center on stealing valuable items, but her methods and targets suggest a deeper, albeit lightly satirical, agenda that challenges simple notions of right and wrong.

In the story, Reinya operates from a secret hideout located beneath the Faminiya store, where she and Chūtarō plan their nightly capers. The narrative follows an episodic structure focused on her various heists, which range from stealing priceless artifacts to exposing corporate greed. Her primary antagonists are a trio of inept police officers from the nearby Dai Edo police station, known as the stupid trio. This group includes a lecherous inspector, a delirious detective, and the clumsy policewoman Nocchi, who serves as Reinya's main foil and is perpetually outwitted in her pursuit of the thief. Reinya consistently evades capture, often making her pursuers unwitting participants in her schemes. Another noted figure is the detective named Keiji, a more sensitive member of the police force who appears to admire the phantom thief, and perhaps her daytime identity, Reina.

Reinya does not undergo significant character development across the series, remaining a consistently clever and successful trickster. Her role is to subvert expectations of both the heist and magical girl genres. Her notable abilities include the use of absurdly over-the-top magical transformations, a mix of charm and wit to infiltrate high-security locations, and a knack for devising elaborate schemes and disguises that allow her to consistently outsmart law enforcement. Despite the frequent and often perilous abuse she subjects her companion Chūtarō to, the mouse always survives, adding to the series' surreal and comedic tone.