Rei operates as Commander Kyoka Iburi's subordinate in the Neo International Defense Force (NIDF) throughout the conflict against the Hadeans. She frequently approaches other characters—including protagonist Shu Aiba and his former squad mates—offering hand-made tea, wine, or Bloody Marys with a condescending and sarcastic tone. This outwardly unserious demeanor masks her deeper significance in the human-Hadean conflict.

She is the biological child of Kyoka Iburi and the Book of the Dead, conceived after Kyoka's exposure to the artifact during the Siren II cruise ship incident years earlier. This lineage connects her directly to the Hadean threat and explains her allegiance. Her role involves active manipulation, such as exploiting Shu Nanao's hidden feelings for Yuno Rukina to sow discord among human forces.

In the climax, her true purpose is unveiled: she commands the Beelzebubs, the Book of the Dead's insectoid agents. Converging to form a colossal body for her, the Beelzebubs transform her into a vessel for the artifact's final objective. In this state, she aims to assimilate all humanity into data, becoming the ultimate arbiter of whether the species deserves evolution or extinction. This transformation positions her as the final adversary confronting Shu Aiba and the Necrodiver forces.