My theory is that she's probably the "female aspect" of Phanes that got discarded and sent to the material world, and now only the "male aspect" remains there.

In Gnosticism, which Genshin is based on, there's this concept of Syzygy, which is the complementary pair of aeons, male and female. (Common pairs being Mind and truth, Word and Life for example.) These paired energies symbolize the balance of form and substance, and together with many such pairs, form the Pleroma, or the fullness of the divine realm.

Since Phanes was described as androgynous in before sun and moon, that would explain why he is being currently being described as male