Kū Shiratori, born Cielo, is a high school student haunted by a lingering emptiness and profound self-doubt, often labeling herself as "empty." Introverted and sensitive to disruptions in her routines or expectations, she escapes into recurring dreams of a nameless prince, exchanging ethereal letters and silent thoughts—a manifestation of her yearning for meaning and connection.

Her fragmented existence originates from a cataclysmic incident ten years prior, which splintered her essence into four entities: Murakumo, Batraz, Claíomh Solais, and Megingjord. The latter fractured further, leaving Kū with the original body while her soul, Valteishia, drifted independently. This schism fuels her perpetual sense of incompleteness.

Residing in Academia City, her path collides with Kyoshiro Ayanokōji—the prince from her dreams—whose presence draws her into clashes involving Absolute Angels: superhuman beings sustaining themselves by absorbing energy through human contact. Kū struggles to reconcile her identity as Cielo with her fractured reality, balancing relationships and duties rooted in her past.

As her body progressively deteriorates, Kū’s physical form disintegrates entirely, only to reemerge reborn as a younger self with fractured memories. Discovered near the European Alps by Kyoshiro and Setsuna, her reunion with Kyoshiro reignites memories of her former life, symbolizing a cyclical resolution to her existential strife. Though physically reset, echoes of her past linger, anchoring her emotional journey of self-discovery and acceptance.

Shy, self-effacing, and conflict-averse, Kū believes herself devoid of intrinsic worth. Yet she demonstrates quiet resilience in facing her destiny and the forces entwined with her origin. Her arc traces a shift from passive introspection to confronting her splintered existence, navigating rebirth’s paradox—a renewal of form that preserves faint traces of the life she once knew.