Who is Marcelle from Story of the Eye and what is their importance?

Marcelle is a young woman who is friends with both Simone and the narrator. Unfortunately, the couple forces Marcelle to participate in their sexual games, which leads to her breakdown. Over the course of the story, Marcelle's mother has her institutionalized, a situation that is quickly ended when the couple breaks her out of the facility. Marcelle later hangs herself.