Accio Quote for the Weasleys says this:
Mafalda: was a Weasley cousin whose character was edited out of the books in favor of Rita Skeeter.
It links to a page on her website that no longer exists, but it can be found on the Internet Archive Wayback Machine:
Mafalda (Goblet of Fire)
I have spoken before now about the Weasley cousin who made it quite a long way into 'Goblet of Fire' before I cut her. I really liked her as a character and did not want to sacrifice her, but she just wasn't doing the job she was supposed to do so she had to go.
Mafalda was the daughter of the 'second cousin who's a stockbroker' mentioned in 'Philosopher's Stone'. This stockbroker had been very rude to Mr. and Mrs. Weasley in the past, but now he and his (Muggle) wife had inconveniently produced a witch, they came back to the Weasleys asking for their help in introducing her to wizarding society before she starts at Hogwarts. The Weasleys agreed to taking her for part of the Summer, including the Quidditch World Cup, but regretted this almost immediately. Mrs. Weasley suspected that Mafalda's parents simply wanted to get rid of her for a while, because she turns out to be the most unpleasant child Mrs. Weasley has ever met.
Mafalda was supposed to convey certain information about the Death Eaters to Harry, Ron and Hermione, because as a nosy, eavesdropping Slytherin who likes to impress, she does not keep her mouth shut when she overhears their sons and daughters talking. Unfortunately, however bright I made her, there were obvious limitations to what an eleven year old closeted at school could discover, whereas Rita Skeeter, whom I subsequently built up to fulfil Mafalda's function, was much more flexible.
The best thing about Mafalda was that she was a match for Hermione. To the latter's horror, Mafalda was highly gifted and a real show-off, so that Hermione was torn between deploring the rule-breaking and longing to join in and beat her.