Harry/Albus Potter Wiki
Cho Chang: "Harry, I never dreamed Marietta would tell... 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 —"
Harry Potter: "Ron's dad works at the Ministry too! And in case you haven't noticed, he hasn't got 'sneak' written across his face!"
— Discussion regarding Marietta's betrayal[src]

Marietta Edgecombe (b. 1978 – 1979) was a witch who was a student at Hogwarts School of Witchcraft and Wizardry, who started between 1990 and 1993. She was Sorted into Ravenclaw House and was the best friend of Cho Chang. In the 1995-1996 school year, at Cho's urging Marietta joined Dumbledore's Army, an organisation taught and led by Harry Potter, despite being reluctant to get on Dolores Umbridge's bad side.

When she became afraid that her mother's job at the Ministry of Magic might be threatened by her membership in the organisation, Marietta betrayed the D.A. to Umbridge. This activated a jinx that caused boils to erupt on Marietta's face, and a Memory Charm was later placed on her by Kingsley Shacklebolt in order to prevent her from fully incriminating her fellow D.A. members.