Harumi Fudo is a rational schoolgirl in 1970s Japan whose disbelief in demons shatters when strange incidents at her school lead to an encounter with the Demon Patrol. Rescued by them from a perilous swimming pool incident, she is plunged into their supernatural world and joins their missions. As the primary human character, she replaces Tsutomu from earlier franchise iterations, assuming a more central role.

Her practicality and level-headedness define her personality, often making her the "only sane woman" who questions the demon team's illogical actions and strives to impose reason on chaos. Though initially reluctant, she develops sympathy for the team and volunteers to help, despite their antics frequently straining her patience. A significant background element involves a ghost named Akira Fudo, who shares her surname and appears only to her. This ghost saves her from drowning and briefly possesses her body to combat a demon, an event she later forgets.

Her adventures subject her to numerous supernatural transformations: becoming a demon, temporarily channeling spiritual entities, and suffering curses that induce dramatic physical alterations, like extreme buttocks inflation following encounters with specific demons. She navigates bizarre dream worlds and reality-warping events, including an episode of uncontrollable mass collapse. Throughout increasingly absurd scenarios, she adapts while upholding her role as the team's moral compass, particularly objecting to plans threatening humanity, such as a proposed mass soul harvest by Heaven and Hell.