Kyōko, previously known as the Equuleus Saintia before her younger sister Shōko inherited the role, is the daughter of Olivia, leader of the Saintias, and shares her family's Okinawan heritage. She mastered the Equuleus Meteor Punch, channeling her Cosmo energy into high-speed strikes.

Her fate shifted during a critical mission to recover the remains of Eris, Goddess of Discord. Seized by the goddess, Kyōko became an unwitting vessel for Eris's resurrection and possession. Corrupted by the goddess's spirit, she emerged as a central antagonist.

As Eris's host, Kyōko wielded divine power to sow chaos, battling the Saintias and Athena's forces, placing her in direct conflict with Shōko. Despite Eris's overwhelming control, flickers of Kyōko's original consciousness and memories persisted, sparking internal struggles. These surfaced visibly during confrontations with her sister.

Kyōko's arc centers on resisting Eris's domination. In a pivotal clash, she wrested momentary control of her body to intercept a lethal blow meant for Shōko, sacrificing herself to protect her sister. This act of love critically weakened Eris's hold, enabling the goddess's defeat. Freed from possession, Kyōko reconciled with Shōko and rejoined the Saintias.

Her narrative is deeply defined by her bond with Shōko, exploring themes of family loyalty, separation, conflict, and ultimate redemption. Their shared history culminates in mutual understanding and renewed dedication to Athena after Kyōko's restoration.