A very thorough description of Mrs. Cullinan is given in Chapter 16, which is when the author describes her experience of working as a househelp for a white family. Note what the following quote reveals about her character and how it conveys a somewhat sardonic attitude towards her:
Mrs. Viola Cullinan was a plump woman who lived in a three-bedroom house somewhere behind the post office. She was singularly unattractive until she smiled, and then the lines around her eyes and mouth which made her look perpetually dirty disappeared, and her face looked like the mask of an impish elf. She usually rested her smile until late afternoon when her women friends dropped in and Miss Glory, the cook, served them cold drinks on the closed-in porch.
Mrs. Cullinan is therefore a character to whom appearances matter more than reality. The way the author describes her as "resting her smile" until her friends arrived suggests that she values what others think about her and how she appears before her friends as being much more important than reality. It also begins to suggest the important role that her friends have and how she is guided by them. It is this that leads to Margaret organising her own exit from this employment.