Mona, a human-dragon hybrid, is the last Yappi survivor in Slaffleaze—a world dominated by intelligent anthropomorphic animals. Her species once served as ancestral guardians of this realm, though that duty has faded by the story's start. Driven by a fierce desire for vengeance against those who decimated her people, she initially distrusts humans but encounters Taku Shimamura after his arrival from Earth. Despite reservations, Mona allies with him against General Zolba, Slaffleaze's tyrannical ruler. Zolba plans to invade Earth—called Capricorn locally—using the cataclysmic Infinity Anchor device, endangering both worlds. Mona's expertise in Slaffleaze and her innate abilities prove indispensable in countering this threat.

As the final Yappi, she embodies independence and resolve, committed to protecting both realms. Though guarded, Mona develops loyalty toward allies and gradually extends trust to Taku during their shared struggle. Her core motivations intertwine halting Zolba's invasion with honoring her inherited guardianship role.

Physically, Mona distinguishes herself as a winged humanoid among Slaffleaze's animal-like inhabitants.