ニア

The female demon who appears after the totem crystal in Kiyoharu's church breaks. She helps avert the immediate crisis affecting the three students and then explains her position as the Fujiwara clan's guardian spirit and the fount of their exorcism powers.

Having slept for decades within the crystal, she is the epitome of unbridled curiosity and doesn't hold back with her speech or her desires. She loves teasing Hinata and Lily in particular, but is easily flustered by praise.

 Prior to her contract with the Fujiwara clan she was a demon who hunted humans for their souls to increase her powers. Eventually she met her match in Kiyoharu's ancestor but at the end of a decisive battle neither side had the strength to claim victory which led to the contract proposal. Each time the Fujiwara clan exorcises evil spirits through sex, these spirits are transferred to the crystal where they are eventually  deconstructed into pure energy which Nia feeds on. On her part Nia can use the energy she receives to teach the Fujiwara exorcist new techniques or creates new "equipment" (usually sex toys) to better assist the Fujiwara exorcist with his work.

Her magical abilities depend greatly on the amount of energy from evil spirits provided by the Fujiwaras. Her speciality is mind-control magic, but is capable of other feats like flight or body modification. She has knowledge of other fields of magic (like Hinata's seances) but isn't necessarily capable of executing them.

It's later revealed in her route that Fujiwara Kiyoharu is the first member of his clan to treat her as a friend or lover. All of his forebears would have sex with her or receive her gifts but they saw her as a demon to be feared and kept their distance.

Her true name is "Eternia Chaosblood". If Kiyoharu uses her true name he can command her to do whatever he likes. Despite this he has little intention of doing so and only appears to call her by that name when they're having sex.