Mayumi Hanazaki serves as a key investigator in the Shounen Tanteidan, a detective team under the Akechi Detective Agency. She tackles cases involving unexpected phenomena, including the sudden emergence of a mysterious black entity within the agency. Her blunt and critical demeanor surfaces during investigations, epitomized by the sharp exclamation, "What is this trash anime?!"—a direct reflection of her unfiltered reactions to inexplicable events.