Marietta Edgecombe continues the lesson that we have learned from both the Weasley and Malfoy families: we are often (but not always) the direct products of our parents. It turns out that Marietta's mother, Madam Edgecombe, works for the Floo office of the Ministry. She has been helping Professor Umbridge police the Hogwarts fireplaces. So Marietta's family is firmly on Fudge and Professor Umbridge's side.
Marietta is a friend of Cho Chang's, and she accompanies Cho to all the D.A. meetings. Finally, after six months of meetings, Marietta decides to rat out Harry and the rest to Professor Umbridge. Using her information, Professor Umbridge tries to ambush the D.A. using the Inquisitorial Squad. Luckily, Dobby manages to tip them off just in time so that only Harry gets caught. He winds up in Professor Dumbledore's office, where Professor Umbridge is waiting with Fudge, two Aurors, Dumbledore, Marietta, and the sign-up sheet from their very first D.A. meeting so long ago.
Hermione has bewitched that sign-up sheet so that anyone who tattled on the D.A. from the list would be punished. And Marietta has definitely been punished: "her face was horribly disfigured by a series of close-set purple pustules that had spread across her nose and cheeks to form the word 'SNEAK'" (27.146). Her awful appearance really throws Marietta off, so she can't stop crying.
What makes her testimony even harder to give (which we find out later) is that Auror (and Order member) Kingsley Shacklebolt works quickly to modify her memory as the meeting starts so that she can't swear to having attended any D.A. meetings before the one planned for that night. Professor Umbridge, frustrated that Marietta has suddenly proved not to be such a great witness after all, actually shakes Marietta hard in front of her boss and a bunch of Aurors. But shaking her doesn't change the fact that Marietta can no longer recall her own involvement with the D.A.
Later, when Cho tries to apologize to Harry for Marietta's behavior, she claims, "She's a lovely person really [...] She just made a mistake – [...] You know, her mum works for the Ministry – it's really difficult for her —" (28.132-5). As Harry points out, Ron's father also works at the Ministry and Ron didn't go to Professor Umbridge behind the D.A.'s back.
On the other hand, Mr. Weasley is firmly on Dumbledore's side, while Madam Edgecombe supports the Ministry's policies. So Marietta doesn't have a good example of appropriate disobedience at home. No wonder she finally decides to side with the Ministry of Magic.
By the end of the novel, we see Cho and Marietta walking past the Hogwarts Express with Marietta wearing a hood to hide her face. This strongly suggests that those nasty purple pustules are still there at the conclusion of the school year. How long is she going to have the word 'SNEAK' on her face, we wonder?