Onpu Segawa emerges as a prominent figure, initially positioned as an antagonistic rival to the main group under Majoruka's guidance. A famous child idol renowned for acting, singing, and modeling, she transfers to Misora Elementary School, initially maintaining a detached and manipulative demeanor. Her early personality is marked by selfishness and a willingness to exploit others to sidestep personal inconvenience, reflecting walls built to keep people distant. A pivotal shift occurs when she overuses forbidden magic to erase memories concerning the main group's witch identities, resulting in a coma. The group sacrifices their magic to save her, forging genuine friendship and securing her official inclusion as a main character starting in "Sharp".

Her path to becoming a witch apprentice began upon encountering Majoruka at a beach during an idol training camp. She excels remarkably, rapidly advancing through exams and skipping three consecutively, earning the recognition "genius witch apprentice." Her magical repertoire includes the transformation incantation "Pretty Witch Onpu-cchi!", the casting phrase "Pururun Purun Famifami Faa!", the stage invocation "Pururun Purun, Suzuyaka ni!", and the royal patraine call "Pururun Patraine!". She is partnered with the fairy Roro and utilizes a purple droplet-shaped crystal ball. Her drive to become an idol stems from a fierce desire to disprove those who ridiculed her mother, a former child idol, fueling her intense work ethic and demand for seriousness from others.

Significant development sees her evolve from initial coldness. Influenced profoundly by Doremi, she dismantles her emotional barriers, becoming considerate, kind, and hardworking, though retaining a capacity for blunt honesty. She contends with loneliness, feelings of inadequacy compared to friends, and a tendency to withdraw when troubled, often masking distress with a smile. Key relationships feature a close, teasing bond with Doremi, whom she credits for revealing true friendship's value; a sensible connection with Hazuki; a frequent pairing with Aiko despite occasional clashes; a strong friendship with Momoko after initial misunderstandings; and a natural, nurturing bond with Hana-chan, whom she diligently cares for despite a demanding schedule. She also aids a shy girl, Karen, in gaining confidence during an audition using magic, sparking a lasting friendship. Her relationship with fan club member Tōru is strained by his career unseriousness, and she finds Alexander T. Oyajiide's obsessive admiration unsettling.

Throughout the series, she navigates the challenge of balancing her idol career, personal life, and responsibilities as a witch apprentice. She grapples with existential doubts about her career path, questioning her future as an idol and exploring acting. Guilt surfaces over perceived insufficient contribution to Hana-chan's care due to her schedule. A notable point is her ability to sing enka, demonstrated through a performance for an old woman (later revealed as Majo Saliban) despite initial reluctance, deeply moving Hazuki. Her name, meaning "Music Note," is reflected in the shape of her head. She is the sole main character lacking round pupils and the only one to break forbidden magic rules multiple times. The 20th-anniversary film "Looking for Magical Doremi" depicts a character named Ryuichi Omiya as her fan, indicating her lasting cultural impact within the narrative universe.