The Hunter’s Moon by O.R. Melling is part of a loosely-knit series called The Chronicles of Faerie, which actually includes several other books that touch on Girls Underground themes, but I chose to profile just one for now.
Gwen is bored with her normal life. While visiting her cousin Finn in Ireland, the two teenagers decide to have an adventure by exploring places that are supposed to be faerie-haunted. During the night, Finn is abducted by a faerie king and Gwen must travel through foreign lands to find her. Along the way, she is helped by a number of companions, human and non-human.
At first her goal is simply to rescue her cousin from the grip of what appears to be her adversary, and she dies indeed thwart him a few times. However, she finds instead that Finn is fairly happy where she is, and that the real adversary is a creature much stronger and more evil. Thus the first adversary, the faerie king, becomes instead another companion, and they all join together to fight this new evil, along with a man who is fast becoming Gwen’s love.