Kuu is quite the normal high school girl who feels as if she's drifting through her existence. She never misses a chance to give letters to the mysterious prince in her dreams and frequently talks to him in her thoughts. In reality, she's an introverted girl, believing that she lacks good qualities. She appears to be easily overwhelmed by situations that deviate from her previous experiences and expectations. Kuu has lost both of her parents, but she does not think of herself as being unhappy because of it.
Her original name is "Cielo", meaning sky, who is the original Absolute Angel researched by Kazuya and his grandfather. During the incident ten years ago, Cielo is split into four pieces, Murakumo, Batraz, Claiomh Solais and Megingjord. In addition, Megingjord is sepparated into two pieces. Kuu is the original body, while Waruteishia is the soul; this explains Kuu's initial feelings that she is lacking something inside.