The Vampire Girl appears as a young lady with long, unkept and orange hair wearing some blue casual shirt and jeans, regardless of what stage she spawns in often making her stick out from the rest of the enemies. She would also have visible fangs hanging from her mouth and often appears at the stage with Vlad the Eternally Cold, most likely since he himself is a vampire.
The Vampire Girl's presence turns all other enemies into Vampires which make them significantly stronger than usual. This is where the Bag of Oregano Relic will be useful since when you have it equipped, it makes the vampires' damages even weaker. The Vampire Girl herself, though, isn't that much of a challenge, she has the same move set as the witches and the pirate lasses.