A Nightmare on Elm Street 3: Dream Warriors
Sister Mary Helena first appears as a mysterious Nun whose sudden appearances and disappearances startle Neil Gordon. She reveals to him the story of how Freddy Krueger came to be, he was the son of a woman named Amanda Krueger who had been gang-raped by the inmates of a mental institution. She called Krueger "The bastard son of a hundred maniacs". She tells Gordon that the best way to destroy Krueger will be to locate his remains and give them the burial they never received. At the end of the film, Gordon follows her and finds that she herself was the woman she had been talking about ("My God! You were his mother!")
A Nightmare on Elm Street 5: The Dream Master
Mary Helena appears in this film as a ghostly, sometimes skull-faced Nun who cannot rest in peace because of her son's evil. Yvonne, friend of the film's protagonist Alice Johnson, must find her remains this time around, so that she can be given enough power to defeat her son.
Mary Helena appears in a flashback in Marvel's Black and white magazine Freddy Krueger's Nightmare on Elm Street. Here she is depicted as a Nun even before she was raped and going by the name of Mary Helena, whereas in the film, her name is Amanda Krueger and it is implied that she became a Nun later in life to find inner peace. It should be noted that this is the first depiction of any kind of origin story for Freddy Krueger and that the depiction of Mary Helena's rape pre-dates the flashback shown in Dream Child.
Mary Helena also appears in Wildstorm's series Freddy Vs Jason Vs Ash: The Nightmare Warriors. Here she organizes the spirits of the Dream Warriors to battle her son's Deadite legions. She and Nancy are the only "members" of the Dream Warriors to speak. She tries to reason with her son to come with her to the afterlife so that God can judge him, but he laughs in her face.