Her first appearance in "The Ungroundable" was sitting in the gym with the vampire children, she later helps Mike and the vampire kids to transform Butters into a vampire.

You were the New Kid and it’s been a few weeks, you were running away from the horrible sixth graders, and then ran into the Gym and slammed the doors. Katie is noticeable as she is seen with Mike Makowski, Larry, Ryan Ellis, and other vampire kids in the school gymnasium. She notes that she would be a psy vampire and then stops talking because she saw you, and her smirk widened.

”Hey, New Kid! Why don’t you come and sit with us? Us vampires will protect you!”

Katie yelled over to you, her sharp fangs gleaming through the Gym’s spotlight, it was… an invitation, or… protection?