Hotaruko Kizuki first appears as Ryo Inasa's cousin inhabiting a remote island. Her demeanor exudes mystery and possessiveness, initially masking schoolgirl-like affection toward Ryo that escalates into an all-consuming, lust-driven obsession. This fixation fuels manipulative and violent conduct, trapping Ryo in a psychological state demanding perpetual acknowledgment and sexual validation.

Her concealed lineage unravels: she is Ryo’s biological mother, while Itsuki serves as her own mother—thus Ryo’s grandmother—and Mizuno as her brother. Bungo, sharing Ryo’s surname, is her great-uncle. Physically, Hotaruko displays heterochromia with one yellow eye and one red, alongside gray hair cropped at ear-length. C-section scars on her abdomen hint at prior childbirth, though the child remains unidentified.

Hotaruko actively engages in the island’s dark cult rituals dedicated to Mizuno, acting as his ally. She lures victims through sexual seduction before orchestrating brutal murders to harvest body parts for Mizuno’s sustenance and mobility, collaborating closely with cult leaders like Chief Priest Komoshi. Her relationship with Ryo intertwines psychological torment with sexual coercion, forcing Ryo to proclaim superiority over lover Ruka and subjecting Ryo to sexual torture fused with emotional manipulation. This culminates in a scenario where Ryo’s compliance becomes essential to escape a fabricated realm of psychological agony.

After Mizuno’s demise, Hotaruko interacts with his corpse through implied cannibalistic acts. The narrative concludes with her seated on Mizuno’s former throne, physically altered by a distended stomach suggesting consumption of his remains, positioning her as his cult successor. Her fate includes surviving an attack by Bungo avenging Itsuki’s death, recovering while bandaged, though her ultimate destiny beyond assuming cult leadership remains unstated.