Lumi Kobayakawa begins as a supportive twin sister to her sibling involved with upperclassman Katsuyuki Touma. A mysterious bacterium from an ancient burial site infects both twins, radically transforming Lumi. The infection twists her personality, replacing supportiveness with consuming jealousy and possessiveness targeting Katsuyuki.

This bacterium also grants the twins terrifying psychic abilities, their strength linked to lunar cycles. Lumi specifically wields telekinesis, the power to kill, and the ability to control others, turning victims into compliant or aggressive slaves. She demonstrates levitation.

Twisted by the infection, Lumi obsessively focuses on eliminating anyone she sees as an obstacle to possessing Katsuyuki exclusively, including her own twin sister, whom she repeatedly targets with lethal intent. Her methods escalate from manipulation to murder, strategically employing her psychic powers to control others and orchestrate attacks against her sister and Katsuyuki. She often displays unsettling, sinister smiles.

Lumi's relentless pursuit drives the central conflict, forcing her sister and Katsuyuki into a desperate struggle for survival. Her motivations fixate solely on claiming Katsuyuki as her sole possession, showing complete disregard for the well-being of others, even former friends and family. The infection amplifies latent jealousy into homicidal frenzy, establishing Lumi as the primary antagonist creating sustained narrative tension.