Corrected entry: When Ash and co. arrive on Shamouti Island, Melody doesn't seem to know who Misty is, hence her asking if she was Ash's girlfriend. Five seconds later, she calls Misty by her name. Yes, Misty is a Gym Leader and she would be famous, but Melody didn't know who Misty was a few moments ago, so this doesn't fly.
Brad
19th Oct 2012
Correction: Even if you know who someone is, that doesn't mean you know everything about them. How else could she have known that they weren't a couple without asking?
Knever
She did ask if Misty was his girlfriend. She did not know if they were a couple or not. She also asked if she was Ash's sister, so it's clear she didn't know who Misty was, but suddenly knew her name.