While in their mother's womb, Sia was growing alongside her twin, Kikyou. However, due to their mother's sudden depletion of mana, both Sia and Kikyou were at risk of dying before birth. In a situation where only one could be saved, Sia used her own mana to absorb all of Kikyou to ensure her twin's survival. Consequently, Sia was born safely, but she lives with Kikyou as a dual personality. In the period immediately following the [Opening], when migration was not yet common, Sia stayed briefly in the human realm for her father's work. One day, while out in a busy district, Sia got lost. That was when she met Guest, who played with her and helped her find her way. Sia wanted to meet Guest again, but she had to return to the divine realm as soon as her father's work was finished and couldn't visit the human realm for a long time. Years later, Sia decides to study abroad at the National Verbana University in the human realm. There, she reunites with Guest, who has also entered as a freshman. Guest does not remember Sia because it happened so long ago. Sia still cherishes the feelings she had for Guest back then, but at the same time, she hesitates to confess because she feels she must find happiness together with Kikyou.

As the dual personality residing within Sia, Kikyou knows everything Sia sees, hears, and feels. She never normally surfaces, but when Sia is in a crisis or conflict, she speaks with her internally. In extreme situations, Sia retreats inward and Kikyou takes control of the body to act. The fact that Sia has a dual personality is something Guest is currently unaware of.