Mayumi Dōjima, a second-year middle school student at Yubiwa Academy, carries a defining childhood memory from her fourth birthday beach trip: witnessing an intensely bright light in the night sky she believed was a unique star. This event sparked her unwavering dream to become an astronaut and "touch" that star. While her family initially encouraged her, growing skepticism culminated in a parental ultimatum demanding she abandon her quest by her fourteenth birthday.

Physically, she possesses long purple hair reaching her waist, often featuring an ahoge, and distinctive purple eyes with star-shaped highlights. She habitually wears glasses to shield her eyes and mask their appearance. Her standard attire is the Yubiwa Academy female uniform: a blue blazer over a white shirt, paired with a long skirt and black tights. For activities with the Pretty Boy Detective Club, she crossdresses, adopting a professional-level masculine disguise that includes shortened hair (except for a longer section on the right side) and the male school uniform.

Initially presenting a gloomy demeanor and self-described "bad personality," Mayumi reacts with anger when complimented on her eyes. Yet, she demonstrates fierce determination, persisting in her decade-long search for the star despite widespread dismissal of her dream. After encountering the Pretty Boy Detective Club, she gradually becomes more assertive and expressive, though she retains a critical view of vanity and superficial beauty.

A congenital condition grants her superhuman eyesight, enabling her to perceive details invisible to ordinary vision, like hidden individuals or camouflaged objects. This ability carries a risk of blindness by age twenty if unprotected, necessitating her glasses as a power limiter. Her vision proved pivotal in multiple cases, including her childhood observation which identified a covert government satellite, later making her a target of surveillance.

Her involvement with the Pretty Boy Detective Club commenced on the eve of her fourteenth birthday. Manabu Soutouin encountered her stargazing on a school rooftop, prevented her accidental fall, and invited her to the club's headquarters. Presenting her star-search request, the club accepted, leading to a helicopter-assisted investigation at the original beach. To comply with the club's second rule ("be a boy"), she adopted a male persona through crossdressing, eventually joining permanently under the alias "Mayumi the Seer."

Key cases featuring her include locating the "dark star" (revealed as a decaying satellite), infiltrating an illegal casino run by a rival school, and investigating thirty-three enigmatic paintings discovered in a secret room during a club-room renovation – a case intertwined with a seven-year-old Yubiwa Academy kidnapping mystery. Her visual abilities consistently provided critical breakthroughs, such as detecting invisible casino technology.

Character development shows her shift from insecurity to confidence. Initially questioning her place in the club due to lacking detective skills or masculine traits, she felt overshadowed by other members' talents. Over time, she embraced her role, becoming more vocal and occasionally haughty. She formed notable bonds, particularly with Michiru Fukuroi ("Delinquent-kun"), sharing candid conversations. Her growth culminated in self-acceptance within the group.

In later narratives, including spin-offs and crossovers, she assumes leadership roles like student council president and participates in missions beyond school, such as celestial observations at the North White Snake Shrine alongside characters from other Nisio Isin works like the Monogatari series. Adult depictions present her in conventional societal roles, contrasting her adventurous youth.