Yuno Rukina is Chief Scientist of the Neo International Defense Force (N.I.D.F.), leading investigations into the origins and nature of the Hadean life forms endangering humanity. Her research centers on the enigmatic "Book of the Dead," an artifact with suspected ties to Hadean mutations and invasions. A close personal relationship with fighter pilot Shu Aiba profoundly influences his motivations and actions during the conflict.

During humanity's first global counterattack against the Hadeans, Yuno suffers severe memory loss, erasing her past except for fractured glimpses of Shu’s name. She harbors a persistent, misplaced guilt over believing she caused his death. A pivotal moment unfolds when Shu seemingly perishes after his mecha fails in battle; in her anguish, Yuno stabs him with the Book of the Dead. This act unexpectedly resurrects Shu, transforming him into the enhanced entity known as the Necrodiver—a key asset in humanity's survival struggle.

Yuno later resurfaces with complete amnesia, retaining only memories predating her N.I.D.F. service. Shu Nanao, a former squadmate of Shu Aiba, discovers her and manipulates her condition by fabricating a romantic history between them. This deception ignites tension when Shu Aiba reunites with Yuno, though her memories of their past bond stay inaccessible.