From City of Hope
In 1997, a bane of Countess Aliara's brood named Urantaran attacked Vivien Delacroix while she slept, creating a child of flesh and spirit. The flesh-half came to be known as Celeste, the spirit-half was born knowing its name to be Orinixia. To some extent, the spirit has based its appearance on Celeste's, so while technically genderless, it appears to be female and refers to itself as such. While Celeste still doesn't know exactly who and what she is, Orinixia knows exactly that. Further, she knows it's her destiny to unseat the weak and indolent Celeste personality, taking full control of the body they currently share. Until then, she bides her time, sculpting Celeste's body and mind to her ends -- as dictated from the distant Court in Malfeas.
In short, Orinixia isn't just some wandering opportunist who took advantage of a random human's weakness. She is Celeste's twin, soul mate, advisor, and "better" half. And she intends, one day, to be all that remains when Celeste is gone.