Kyouka Iburi emerged from the ashes of the Siren II cruise ship disaster, surviving as a child stranded on a Pacific rock. This ordeal brought her into contact with the Book of the Dead, an artifact entwined with the Hadeans’ influence, which later shaped her ambitions. Ascending to command the Neo International Defense Force (NIDF), she publicly championed humanity’s defense while secretly molding the organization into a veiled recruitment front for the Beelzebubs—agents tasked with selecting successors capable of opposing the Necrodiver and deciding mankind’s destiny.

Her bond with the Hadeans intensified after giving birth to Rei, a child born from her exposure to the Book of the Dead during the Siren II tragedy. Rei’s eventual leadership of the Beelzebubs and threat to digitize humanity bound Kyouka irrevocably to the Hadeans’ schemes. Though her betrayal initially lacked clear motives, revelations exposed her manipulation of prophetic visions from the Book, weaving pivotal events like Yuno Rukina’s induction into the NIDF and Shu Aiba’s death-and-resurrection as the Necrodiver.

Kyouka’s cold tactical calculus defined her reign. After a timeskip, she permitted Hadeans to consume 70% of Earth, exploiting the chaos to further her ends. Her complex dynamic with Souma Masaki, a devoted lieutenant, hinted at unspoken personal stakes beneath her calculated exterior. Rooted in her origins as the Siren II’s lone survivor, her evolution into a central antagonist exemplified the Book’s corrosive power, culminating in a final gambit pitting Shu’s Necrodiver form against the Hadean entity Abhoth to decide mankind’s survival.

Revised narratives in "Kiss Dum R" and OVAs deepened her role, detailing her covert experiments to transform Shu’s comrades into semi-Necrodivers and her shadowy orchestration of their clashes with him. Despite minimal early presence, her legacy as the architect of humanity’s resistance—and Rei’s apocalyptic design—anchored the series’ climactic reckoning.