Susan in The Mayor of Casterbridge is a young, attractive, unhappily married woman with a baby daughter. We don't learn much about her, so she remains a somewhat mysterious figure, but we do know she doesn't expect much out of life and is pragmatic.

Her acceptance that life is limited and isn't likely to offer her much is communicated by the narrator, who says of her,

she had the hard, half-apathetic expression of one who deems anything possible at the hands of Time and Chance except, perhaps, fair play.

In other words, young as she is, life has already let her down and left her feeling that the world is not on her side. A key part of this is that her marriage is strained. She and husband say walk side by side without touching and pay little attention to one another. If Susan talks, it is to her baby daughter.

Susan's practical, down-to-earth nature is revealed at the fair. She is not interested in the games or excitement and decides they should eat at the tent serving furmity. Furmity is a cornmeal dish made with milk, currants, and raisins that is cheap and nourishing, a practical dinner choice.

When her husband gets drunk by having rum repeatedly added to his bowls of furmity and puts her up for auction, her pragmatic nature is such that she does not argue when she is sold to a sailor. She goes willingly, saying to her husband,

Mike ...I've lived with thee a couple of years, and had nothing but temper! Now I'm no more to 'ee; I'll try my luck elsewhere.

She then unromantically throws her wedding ring back at him. Susan is a woman who has been crushed down by a marriage to a bad-tempered man, but she has enough spirit left to take the opportunity for escape that is offered her by her husband's callous action.