A shapeless silver light shoots from the tip of your wand toward the dementor. It manages to stop the creature from feeding on the poor girl, at least for now, but your incorporeal patronus won't make it go away completely.

The girl is deathly pale and trembling violently. She suddenly drops to all fours and throws up. She eventually looks up at you with wide, frightened eyes. "W-who are you? Why am I here?" she asks.

Then she notices the bright silver light. "Where is that light coming from?"