Setsuna exists as the Absolute Angel Claíomh Solais, bound to Kyoshiro Ayanokōji as his combat partner and protector. Her origin traces back to the cataclysmic fragmentation of the original Absolute Angel Cielo ten years before the main story, splitting into Murakumo, Batraz, Claíomh Solais, and the divided Megingjord. She operates under the title "Setsuna of the Moment of Dreams."

Characterized by emotional reserve and unwavering devotion to Kyoshiro, Setsuna rarely expresses personal desires. She habitually responds to directives with "Would you prefer that, Kyoshiro?" and acts solely according to his will. Her exceptional competence in domestic tasks leads others to perceive her as flawless or princess-like. A distinctive physical trait is the ribbon bow with bells, gifted by Kyoshiro and tied to her headdress; this serves as her focus item for manifesting mecha abilities. Unlike typical pilots, she controls mechanized armor components externally, phasing in and out of her angelic form during combat. Her Absolute Angel symbol is located on her right arm.

Setsuna's development centers on evolving emotional awareness, particularly concerning her bond with Kyoshiro. Initially suppressing personal feelings, she experiences jealousy as Kyoshiro grows closer to Kū Shiratori. This culminates in emotional turmoil when Kyoshiro confesses his love for Kū, marking the first failure to restrain her true feelings. This conflict manifests in the OVA (Special Three) through her imitating Kū's behavior to gain Kyoshiro's affection.

Ultimately, Setsuna achieves acceptance. She acknowledges Kyoshiro's love for Kū and supports his mission, symbolizing her emotional maturation by cutting her hair. She accompanies Kyoshiro to locate Kū's reincarnated form near the European Alps, where she formally relinquishes her claim on him and bids farewell. This act completes her journey from unquestioning obedience to autonomous emotional resolution.