I was probably about 2/3 of the way through Possess by Gretchen McNeil before I was sure it was a Girls Underground example. Not that it starts slow – in fact, it jumps right in with Bridget, 15, helping the Monsignor at her Catholic school with an exorcism. Bridget has recently started hearing the voices of demons (after her father’s murder), and the Monsignor is helping her use this “talent” to banish them from this world. A second priest arrives from the Vatican to help, although Bridget is wary of him. A large portion of the story is Bridget just trying to deal with all of this, in addition to the usual teenage stuff like avoiding the guy who has a lifelong crush on her. Though there is an excellent scene of an exorcism in a doll shop, with a lot of creepy imagery of all the dolls acting as one under the control of the demons – terrifying for any of us who, like the protagonist, find dolls automatically disturbing.
However, it eventually becomes a classic GU story – unfortunately, describing exactly how would introduce too many spoilers. Suffice it to say that there is a demon adversary and a human minion who is actually a companion who betrays her. She must rescue a family member. And in the end, the only thing that will save them all is if Bridget accepts her destiny, in a one-on-one showdown with the adversary.