Non-canon Weasley cousin.
Rowling had created the character of a female cousin to the Weasleys for Harry Potter and the Goblet of Fire; however, that character disappeared in rewrites.
On her website, Jo gives some more insight into the character: Her name is Mafalda, she is the daughter of the nameless accountant second cousin (a Prewett cousin through Molly – PS6), and she would have been a Slytherin (JKR). Here’s what Rowling has to say in an interview about Mafalda:
Q: Last time we spoke you said there’d be a Weasley cousin. It didn’t appear. You’ve deceived me!
A: It got pulled. Sorry about that. What happened on Book Four, and one of the reasons why it was easily the most difficult to write, which had absolutely nothing to do with Harry being famous or me being famous, was that for the first time my plan fell down. I got halfway through and realised there was a huge gaping plot hole. The two ends just didn’t meet. It was entirely my own fault: I should have had the sense to go through it very carefully before I started writing. So I had to do an enormous amount of unpicking and in the process I’m afraid the Weasley cousin disappeared.
Q: Will we see her again?
A: Possibly. I really like her as a character but it’s quite a complex plot I’m dealing with so I’m not sure that she’ll fit anywhere else. She’ll be the “character that might have been.” (Nr)