Girly TeenGirl is a female teen sponge who had just moved to Bikini Bottom from Farawayville in the episode "The Slumber Party." Pearl and her friends all think she is SpongeBob in disguise, so they throw tomatoes at her. She looks a lot like SpongeBob and it is shown that she isn't SpongeBob because as she is running away, SpongeBob and Mr. Krabs immediately drive up and have no idea who she is. Her part ends when she screams, "I am so moving back to Farawayville!" At that point, Mr. Krabs says, "'Who was that?'" whereupon SpongeBob replies by saying, "I don't know...but she is ugly!"
Ironically, though she looks a lot like SpongeBob, SpongeBob thinks that she is ugly.
Because Pearl thinks that she is SpongeBob, this means that Pearl does not know who Girly TeenGirl is. So it is unknown why Girly TeenGirl showed up at Pearl's slumber party, unless perhaps one of Pearl's friends invited her.
In the Dutch version, she is called Spongy Betty.
She may actually be the same species as SpongeBob.
She could possibly be distant relative of Spongebob.