Meimi Haneoka attends St. Paulia's Academy, a Catholic private school, presenting as a typical fourteen-year-old with long orange hair and deep blue eyes. She wears a school uniform featuring either a pink jacket with a green neck bow and yellow skirt or a summer pink pinafore. Academically gifted in physical education yet challenged by mathematics, her ordinary student life starkly contrasts with her secret identity as the phantom thief Saint Tail. At night, she returns wrongfully taken items using stage magic techniques—never supernatural means—honed from her father, a professional stage magician, and inspired by her mother, a former phantom thief who retired upon marriage. Her pet hedgehog Ruby accompanies her.

As Saint Tail, she ties her hair in a sleek ponytail with a black ribbon and dons a magician-themed ensemble: a sleeveless white collared shirt, high-necked black vest, long gloves, pinkish-black leggings, boots, and occasionally a top hat. The manga renders this outfit pure black, while the anime introduces pinkish tones. Her transformation begins with the prayer "Oh Lord, forgive me, for I use no gimmick or trick," followed by a ritualized "One! Two! Three!" count, materializing her costume, magician’s wand, and symbolic playing cards. She employs illusions like blinding lights to evade pursuers, projecting doves or cards as diversions, and directing sentient balloon clusters for transport or deliveries. Her signature prelude to major tricks is the catchphrase "It’s showtime."

Seira Mimori, a nun-in-training and Meimi’s closest friend, enables Saint Tail’s missions by gathering requests through church confessionals and providing logistical aid, framing their actions as divine will. Saint Tail publicly challenges classmate and amateur detective Asuka Jr. by sending advance heist notices via inventive methods—such as messages in his lunchbox or on floating balloons. Their dynamic shifts from rivalry to mutual romantic interest, though Meimi fears his hatred if her identity surfaces. School interactions involve frequent arguments, contrasting his admiration for Saint Tail’s altruism.

Throughout her double life, Meimi’s initial confidence wanes as she emotionally withdraws, dreading exposure—especially when Asuka Jr. grows suspicious. The conflict peaks when antagonist Rosemary forcibly unmasks her to Asuka Jr. and abducts him. Rescuing him, Asuka Jr. catches Meimi mid-fall, symbolically "catching" her as thief and person, and forgives her deceptions. This ends her Saint Tail career. Eight years later, she cuts her hair short, marries Asuka Jr.—now a private detective continuing her mission of aiding others—and lives privately, her past as Saint Tail undisclosed.