Mademoiselle Reisz is an elderly pianist. Described as homely, with a small face and a bad taste in clothing, she is viewed as temperamental and disagreeable because she is argumentative and self-assertive. She never works to please others, being a prime example of a woman who goes against society’s norms. She is frequently offensive and disagreeable, but solely because she doesn't care about others’ opinions of her. 

Her assertive and independent personality influences Edna, especially through music. Edna is deeply affected by Reisz’s piano playing and befriends her while staying at Grand Isle. Reisz, who understands Edna’s emotions, claims that Edna is the only one worth playing for. After returning home to New Orleans, Edna often comes to visit with Reisz, who offers her intellectual conversation and gives her news about Robert Lebrun. Because Reisz is a musician and artist, she teaches Edna that art-making takes a brave and courageous soul. She also compares Edna to a bird, and cautions her that she will need strong wings to rise above tradition and prejudice. 

Reisz’s home is also the place where Edna learns of Robert’s love for her. Incidentally, it is also where Edna and Robert accidentally meet after he returns from Mexico. Reisz acts as a catalyst for their love; Robert writes letters to her, and Reisz allows Edna to read Robert’s letters. Before Edna drowns, she remembers Mademoiselle Reisz’s words: “The artist must possess the courageous soul that dares and defies.” Edna imagines that Reisz would laugh at her if she knew that Edna was choosing to take her own life.