Mrs. Moss, who is Mr. Tulliver's sister, is the representative of a world that we don’t see a lot of in The Mill on the Floss: that of extreme poverty. Even after the Tulliver family goes bankrupt, they never experience the level of poverty that the Moses live every day. Mrs. Moss thus plays a crucial role and in the social fabric of the book. During this period, industry was creating as many, if not more, poor people as it was rich people. Lots of people were stuck in low-paying factory or agricultural jobs, were severely in debt, and had too many mouths to feed in the family.
The Moss family is definitely large, in debt, and very poor. They also live in a depressed town that is nearby, but somewhat hidden from the respectable and prosperous town of St. Ogg’s. The Mosses and their home are the sort of hidden, dark underbelly of Victorian industrial society. It is no mistake that Mrs. Tulliver "forgets" to contact the Moss family during the Tulliver’s bankruptcy crisis. The Dodsons view the Mosses with a degree of contempt. Even the kind-hearted Mr. Glegg originally says that the Moss family ought to pay their debt to the Tullivers, even though paying it will be very hard for them, and even though the Dodson clan isn’t offering the Tullivers as much financial help as they could. Mr. Glegg only changes his tune when Tom intervenes and decides to uphold his father’s compassion to his sister.
This is the other important role that Mrs. Moss plays in the narrative: that of a sister. Mrs. Moss is frequently drawn in parallel to Maggie, the book’s other notable sister. The two even share "dark" physical features. Mrs. Moss is the catalyst, or the cause, of Mr. Tulliver’s plea to Tom to take care of his sister:
It had come across his mind that if he were hard upon his sister, it might somehow tend to make Tom hard upon Maggie, at some distant day, when her father was no longer there to take her part. (1.8.55)
Overall, Mrs. Moss is a highly sympathetic and very sad character who seems beaten down by life and who expects to be treated badly. Despite all her problems, Mrs. Moss is a highly compassionate individual who loves her brother and her niece Maggie a great deal. Mrs. Moss often provides comfort to Maggie when others fail to do so.