Mamoru Shimesu operates as both business associate and assistant to Ayaka Kisaragi at their paranormal investigation company. His family's multigenerational alliance with the Kisaragi family mandates his support for Ayaka, despite her rejection of traditional family practices. Functioning as secretary and housekeeper, he oversees administrative operations through computer systems—managing client records, case leads, financial tracking, and outstanding debts. His duties extend to domestic responsibilities including cooking, laundry, and household maintenance.

Ayaka's habitual sake overindulgence causes frequent late awakenings, requiring Mamoru to implement creative morning solutions—typically strategic placements of multiple alarm clocks throughout her bedroom. This operational intervention sustains company functionality despite profitability impacts from Ayaka's habits. Mamoru's skills encompass computer operation, investigation coordination, communications management, and culinary preparation. He maintains the company headquarters within Ayaka's Shinjuku family residence in Tokyo, facilitating daily operations amidst supernatural casework involving ghosts, demons, vampires, and rival paranormal investigators.