Order of the Phoenix

Marietta was first introduced to us when Cho Chang brings her, somewhat unwillingly, to the first meeting of Dumbledore's Army. She is present at the meeting at the Hog's Head, but says nothing, and we do not even learn her name, she is "Cho's friend".

We see her again at the first full meeting of Dumbledore's Army; here, she and Cho are practicing the Disarmament jinx, and Cho is so flustered by Harry's presence that she ends up setting Marietta's sleeve on fire.

Shortly before the Easter break, Professor Umbridge learns about the existence of Dumbledore's Army. Harry, forewarned by Dobby, manages to engineer the escape of the rest of the group, but is himself caught. In Professor Dumbledore's office, Harry finds that Marietta had reported the existence of Dumbledore's Army to Dolores Umbridge. (It is only at this point that we learn her name; Professor Umbridge introduces her as part of her exposition of what has been going on.) As a result of this betrayal, a charm that Hermione had set on the parchment that all DA members had signed, had covered Marietta's face in purple pustules that spelled the word "sneak."

Marietta's blemishes make it very hard for her to show her face in Dumbledore's office, particularly as one of those present is Cornelius Fudge, the Minister for Magic. With her robes up around her face, her responses to questions put to her by Umbridge are answered generally by whimpers and gestures. Before she can actually give much testimony, Kingsley Shacklebolt, there as one of the Auror guards Fudge has brought with him, modifies her memory surreptitiously so that she no longer remembers that the DA had been meeting regularly. Marietta's changing her story almost incites Umbridge to violence against her. Harry recognizes the glazed look in her eye as being a symptom of memory alteration, though it seems nobody else there, with the possible exception of Dumbledore, does. Though she remains in Dumbledore's office until after Dumbledore's departure, Marietta plays no further part in affairs, simply standing there.

Afterwards, Harry and Cho get into something of a fight about Marietta, with Cho saying that Marietta is basically a good person who had made a mistake, that she had felt she had to because her parents work in the Ministry, and that Hermione's charming the parchment to do that to Marietta's face was sneaky. Harry, on the other hand, points out that Ron's father also works in the Ministry, and he isn't going around with the word "Sneak" on his face. He goes on to say that he thinks Hermione's charm was brilliant. This fight basically ends their relationship.

On the Hogwarts Express back home, Harry sees Cho and Marietta walk past; Marietta has apparently not managed to hide the word on her face, as she is wearing a balaclava.

Half-Blood Prince

It is mentioned in passing that Marietta's face still showed signs of the purple pustules, though she tried to cover them with make-up.