Berenice is a foil for Frankie in another way. She represents all Frankie has to learn and know. Firstly, she knows about love, sex and relationships, not necessarily in that order. And secondly, she knows about the real world, about the harsh realities of racism and how it divides people. She serves as a reality check for the delusional Frankie, always questioning Frankie's suspect motives and explaining Frankie's feelings to her, as if she understands the workings of the girl's mind better than Frankie does herself.
Berenice helps flesh out the novel's theme of bifurcation: the separation between two entities. She is herself split. She has one dark eye and one glass blue eye, thus her physicality has both black and white attributes. She discusses with Frankie what it means for a black person to be trapped in a white society. And we see how well she must understand this, what with her own body divided as such.
Considering that the novella was written in the 1940s, it is significant to recognize that a close, empathetic relationship between a white girl and a black woman would have been provocative. This novel seeks to break down stereotypes about black people and to try to give a wide audience a certain understanding and appreciation for the African-American struggle against oppression. Berenice's wise and likable character aids in this message of acceptance for people's differences, giving mid-20th century readers a chance to accept someone their prejudices might otherwise dismiss.