Seiji Hayami is a private detective dedicated to investigating the criminal syndicate Panther Claw. His lifelong mission stems from childhood trauma: the murder of his father, Hayato Hayami, by Sister Jill. This event fuels his relentless drive to dismantle Panther Claw. His path crosses with Cutie Honey during an investigation when she aids him against a Panther Claw kaijin. Recognizing her value, he becomes her informant, unaware of her civilian identity as Honey Kisaragi.

Throughout the series, romantic feelings blossom between Seiji and Honey Kisaragi. Their relationship deepens, culminating in the birth of their daughter, Seira Hayami, near the story's end. Three years later, he marries Honey, solidifying their family. This specific adaptation uniquely establishes Seiji as Honey's official romantic partner.

Seiji exhibits a serious and intensely driven personality, especially concerning Panther Claw cases. His pursuit of justice is rooted in vengeance for his father's killers. Yet, he demonstrates significant emotional growth, evolving from a single-minded avenger into a protective partner and devoted parent. His sharp investigative skills prove vital in countering Panther Claw's schemes, though he depends on collaboration with Honey for direct combat.

In the finale, Panther Claw threatens his daughter, prompting Seiji to fiercely embrace his role as a protector. This reinforces his dual commitment: safeguarding his family while continuing his mission against Panther Claw.