I read The Bronze King by Suzy McKee Charnas several years ago, before this blog. While I recognized it as a Girls Underground story, I did not remember how perfectly it fits the archetype. And not only because the monstrous adversary actually does dwell underground, in the subway tunnels.
Valentine (or Tina, or Val, or Vee) lives in New York City with her mother, who is mostly preoccupied with her job and meeting new men. One day Val feels an explosion near a subway entrance, and shortly after starts noticing things disappearing – at first small things, but then more significant ones, including a large bronze statue in the park. Unnerved and unsure of what to do, she makes a wish in the manner her grandmother taught her, and soon after an itinerant violinist named Paavo shows up who seems to know all about the strange happenings. He tells her they are due to a force trying to come into this world and devour it (which he calls a kraken, though it is not literally one). They must find the missing statue, which was made to guard the entrance against such monsters. She is also helped by a somewhat annoying boy named Joel, who just sort of falls into being a companion.
Val discovers that she has the key to the door where the statue is being hidden, but is grounded by her mother and cannot leave to use it. Joel tries himself instead of bringing the key to Paavo, which results in him and the key being captured and held by the kraken in the subway tunnels. The kraken’s minions, a group of street thugs, keep getting in the way and attacking them. Val and Paavo go to her grandmother for help, who tells them Val must get the key herself, but leave Joel behind. Scared and looking for a way out of this responsibility, Val goes home briefly but must run away again. Eventually she frees the statue but must face the kraken alone to save the world.