Ghoulia Yelps is 16 years old in monster years and is the daughter of the zombies. Ghoulia is the only main monster who doesn't speak English, or, not really many understand her. Which, is ironic due to that she is the smartest monster in the school. Ghoulia also seems to be the only monster who isn't really connected to her parents.
Ghoulia's smarts seemed to be used against her at times when Cleo asks her to do her homework, au projects and gets the credit instead of Ghoulia.