Kū is a rather bland high school girl who feels as if she is drifting through her existence and she always describes herself as being "empty". She never misses a chance to give letters to the mysterious prince in her dreams and frequently talks to him in her thoughts and, sometimes, even out loud. In reality she is a shy(almost stereotypically so) and self-deprecating girl, believing that she lacks good qualities. She appears to be easily overwhelmed by situations that deviate from her previous experiences and expectations.

Her original name is Cielo (チェーロ Chēro), meaning sky, who happened to be the original Absolute Angel created and researched by Kazuya and his grandfather. During the incident ten years prior to the main story, Cielo is split into four pieces; Murakumo, Batraz, Claíomh Solais and Megingjord. In addition, Megingjord is separated into two pieces. Kū is the original body, while Valteishia is the soul; this explains Kū's initial feelings that she is lacking something inside. In the end of the series, her body crumbles and she reincarnates as a smaller version of herself. Prior to the end of the last episode, she has dreams of the four Absolute Angels. After the credits of the last episode, Kyoshiro and Setsuna find her near the European Alps, and when she sets her eyes on Kyoshiro she instantly remembers her past life's memories.