Beatrice Hunsdorfer
Beatrice is the central character around whom the play centers. She is a single mother abandoned by her husband years ago, which has led to a profound distrust of men. She remains in the house where she grew up and has become increasingly withdrawn over the years. Beatrice still grieves the loss of her father, whom she had to commit to a sanatorium years prior. The world has inflicted much pain on her, and she often directs her anger toward those around her, especially her two teenage daughters. Beatrice has always longed for popularity but has always been an outsider. She desperately wishes to escape her current life and often dreams of a better future.

Ruth Hunsdorfer
Ruth is Beatrice's elder daughter. She suffers from epilepsy, with seizures triggered by anxiety or stress. Ruth is somewhat promiscuous and deeply concerned with her appearance. She constantly frets about her younger sister, Tillie, embarrassing her at school. Ruth's relationship with Tillie is unpredictable; she often mocks her but is quick to boast, ‘‘That's my sister,’’ when Tillie wins the science fair and gains some school fame. Ruth has a volatile temper and is unafraid to talk back to her mother. Her attempts to hurt her mother drive much of the play's action.

Tillie Hunsdorfer
Tillie is Beatrice's younger daughter. She is ostracized at school and teased by her peers. Tillie is highly intelligent, and her teacher, Mr. Goodman, nurtures her interest in science. The title of the play derives from Tillie's experiment for the school science fair. Tillie is a dreamer who longs for a better world. She is quiet, thoughtful, and somewhat awkward, often facing criticism from her mother for these traits.

Nanny
Nanny is the boarder residing in the spare room. She is nearly blind and deaf and can barely walk even with a walker. Nanny's daughter has relinquished responsibility for her care.

Janice Vickery
Janice is Tillie's main rival at the science fair. She delivers a gruesome yet humorous presentation where she describes boiling a cat and assembling its skeleton. Janice's speech provides comic relief just before the play's dramatic climax.