Rouran is a mysterious girl who appears suspended in air, radiating light and capable of striking targets with beams. She suffers complete amnesia, forgetting her identity, origins, and purpose. This memory loss includes why the presence of Gousen (metal fairies) or Kyoushi (reanimated corpses) triggers an involuntary transformation into an avenging angel. These reactive transformations link her abilities directly to the conflict between the occult society Shiromorishuua and the peace-keeping organization ASY.

In the settlement Tokyo Arcadia, a haven for expatriates and dissidents, Tetsuya provides support and watches over her. His protection offers stability as she navigates her confusion and existential questions. The narrative explores her connection to the past shared by Dr. Masahiko Mikogami, ASY's founder, and the Shiromorishuua group, suggesting deeper ties to the central conflict. Her character arc revolves around seeking answers about her existence while wrestling with the destructive power unleashed during her angelic transformations.