Catherine Sue, 29, serves as editor-in-chief of Monthly Moso Science, having recruited staff like Tarō J. Suzuki and Jirō Tanaka. Her past identity was Four Tune, a child prodigy scientist whose early acclaim shattered at age 15. The scientific community publicly rejected her theory of the Mo Continent's existence during an unauthorized press conference, discrediting her. This event fueled her future actions.

Already possessing a MOPart artifact as Four Tune, she could transform into an anthropomorphic fox-like Motarian form, reflecting her ancestral role as advisor to the Mo Continent's king. Adopting the Catherine Sue identity after her fall, she founded the magazine to locate other reincarnated Motarians and gather MOParts. Her true goal was raising the sunken Mo Continent to activate the doomsday weapon Ra Mo, eradicating humanity and fulfilling her ancestors' plan.

Throughout the series, Catherine manipulated her staff into collecting MOParts, concealing both her Motarian heritage and distinctive "お"-shaped marking. Her Motarian form demonstrated significant physical prowess, nearly defeating Jirō in combat upon its revelation. Her plan culminated as the assembled MOParts raised the Mo Continent, enabling Ra Mo's activation.

At the climax, Catherine achieved partial success, proving the Mo Continent's existence and regaining scientific credibility. However, confrontations with the protagonists, especially Jirō—the reincarnated Mo prince—led her to abandon the genocide objective. She continues researching the Mo Continent peacefully, maintaining her human identity. Catherine possesses aubergine hair and cyan eyes.