Mary Seton is Mary Beton's BFF. After a terrible meal at the women's college Fernham, they confab up in her room about how women have always been poor while men have been rich.
We don't learn a lot about Mary Seton herself. She lives at the women's college, but isn't a student, so we might assume that she is a professor or administrator there. If Mary Beton represents the independently wealthy woman writer, Mary Seton is the female intellectual who works for her money at a university. She's badly fed, and probably badly paid.
Weirdly, we do learn a bit about her mother: she gave Mary Seton a happy childhood in Scotland and bore thirteen children (1.28). Why does Woolf drop this fun fact in? Well, we think it probably has something to do with Woolf pointing out that Mary Seton has obviously made different life choices—and that's okay. Not every woman needs to have thirteen kids. In fact, some women might choose to have no kids. They choose their choice!