Eva Mattes was in four Fassbinder films in all; The Bitter Tears of Petra von Kant, Wildwechsel, Effi Briest and In a Year of 13 Moons. This is a great look here, the dippy 19thC Century girl dreamer; nobody works the swing better.
Wildwechsel, or Jail Bait, or Wild Game (1973) isn’t among the easier of Fassbinder’s films to see or track down. A print shown at MOMA in New York has been described as a dirty faded unrestored mess. Still, and especially when using its US handle ‘Jail Bait’ Wildwechsel still attracts interest, just for its sexual themes alone. It’s one of Fassbinder's darkest movies, and that is indeed saying something.
In Effi Briest by Fassbinder, she does what you see her doing in this picture, she plays a young friend of Effi, with the sort of distant expressions and forlorn dips in mood that were common among all 19th century German girls.
If you are bothred by anything in watching Fassbinder's Effi Briest, it's the casting of Hannah Schygulla as a teenager. But having beautiful Eva Mattes as Effi’s friend Hulda makes Hannah Schygulla seem more like a girl, certainly in her company.