Kotori Itsuka is Shido Itsuka's adoptive younger sister and simultaneously commands the secret organization Ratatoskr from the aerial ship Fraxinus. Five years before the main events, expressing fears of abandonment led to her transformation into a Spirit by the mysterious Phantom. Phantom instructed her to kiss Shido to save his life, inadvertently activating his power to seal Spirit abilities. Both subsequently had their memories suppressed. Kotori later reclaimed her Spirit powers during a confrontation with Kurumi Tokisaki, leading Shido to reseal her powers and restore their lost memories.

Physically, she possesses long coral-red hair typically worn in twintails secured by ribbons, red eyes, and stands 145 cm tall. Her ribbon color dictates her human personality: white signifies a gentle, dependent "little sister mode," while black activates a stern, strategic "commander mode" required for leading Ratatoskr. Her Spirit form manifests through the Astral Dress "Elohim Gibor," a white kimono with translucent elements and horn-like adornments, initially reflecting her ribbon-based demeanor. However, prolonged Spirit form use risks triggering a third, uncontrollable personality marked by destructive aggression and battle lust. Following significant emotional events, she adopts both ribbons simultaneously, symbolizing the integration of her dual personas.

Codenamed Efreet, Kotori wields the Angel Camael, a halberd that transforms into the cannon Megiddo. This weapon channels her pyrokinesis to generate intense flames for precision attacks or wide-scale devastation. Her abilities include high-temperature projectiles and a potent regeneration factor that heals physical injuries and repairs her Astral Dress through flame-based restoration. Due to destructive impulses linked to her powers, she activates her Spirit form only during extreme emergencies. After her powers are sealed, she retains compatibility with Ratatoskr's Realizer technology, enabling limited barrier creation and tactical support.

Kotori's development centers on reconciling her identities. Early portrayals show a sharp divide between her affectionate white-ribbon self and her abrasive commander persona, later revealed as a leadership facade. The resurgence of her Spirit powers and subsequent resealing deepen her trust in Shido. Discovering Ratatoskr's contingency plans, such as a kill satellite targeting Shido, highlights her burden of command. Her eventual use of combined ribbons reflects acceptance of both vulnerability and strength, stabilizing her role within Ratatoskr and her relationship with Shido.