Jill is a minor nine-year-old freak. She has the power of essentially enchanting people when she sings.
She was introduced in Lies , being dragged from her home and threatened by the Human Crew , who put masking tape around her mouth and scrawled 'FREAK' on her forehead. They left her in the street where she wandered around for a few hours until falling into the hole that Brittney/Drake dug from their way out of the grave.
Nerezza refers to Jill as "Siren" and, at the very end of the book, when Sam was on the verge of destroying Drake Merwin, Nerezza made her sing and distract everybody, giving Drake time to regenerate. Her singing was then drowned out by the sound of the Brattle-Chance children arriving by helicopter.
Jill's ultimate fate is unknown. However, given that Gaia never attempted to make use of her powers, it seems likely Jill died between Lies and Light.