Nancy is a good country girl. Like her sister (see Priscilla's "Character Analysis"), she's been brought up to be useful. The citified Miss Gunns sneer at her hands, which "bore the traces of butter-making, cheese-crushing, and even still coarser work" (1.11.9), but Nancy is not ashamed of her competence or her country speech. She lives by an "unalterable little code" (2.17.26), which governs her moralistic but just approach toward life. She has high expectations of herself, and she expects equally upright behavior from her surroundings.
When we first meet Nancy, she's worried about her upcoming run-in with Godfrey:
It was very painful, when you had made it quite clear to a young man that you were determined not to marry him, however much he might wish it, that he would still continue to pay you marked attentions; besides, why didn't he always show the same attentions, if he meant them sincerely, instead of being so strange as Mr Godfrey Cass was, sometimes behaving as if he didn't want to speak to her, and taking no notice of her for weeks and weeks, and then, all on a sudden, almost making love again? (1.11.1)
This little interior monologue actually tells us a lot about Nancy. First of all, she's not quite honest with herself. She says that she's "determined not to marry" Godfrey, which might suggest that she doesn't like him. Except she goes on to wonder why he pays attention to her sometimes and then totally ignores her the next day—exactly as if she is in love with him, after all.
If she's in love with him, then, why won't she marry him? He lives "a bad life" (1.11.2) that doesn't stand up to her expectations. When Godfrey seems to be heading down the wrong path, she holds herself away from him, hinting as directly as she can that she doesn't want to marry a man whose behavior can't be trusted or relied upon: "it 'ud be better," she says, "if no change was wanted" to his behavior (1.11.87).
That's a reasonable expectation, but Nancy has some funny ideas. She believes, for example, that sisters should be dressed alike (actually fairly common in the 19th-century), and she promises that she'd say the same even if Priscilla insisted on something hideous, for "who shouldn't dress alike if it isn't sisters? Would you have us go about looking as if we were no kin to one another—us that have got no mother and not another sister in the world" (1.11.22). The problem is that, while Nancy looks pretty in anything, Priscilla is, in her own words, "ugly" (1.11.13). It would be easy to suspect Nancy of being a little mean—or, as the narrator says, of having a little "malicious contrivance" (1.1..26)—but she's not at all. She really does want to dress the same as Priscilla, because that's what she believes sisters should do.
This same code means that she devotes herself to Godfrey exactly as a wife should, but it also means that she can't even allow him to adopt Eppie. As she thinks to herself, she had been "forced to vex him" by refusing to adopt Eppie. It's weird to use the word "forced," because of course she wasn't really forced—she simply refused.
It's hard to figure out exactly what the narrator thinks of Nancy. When Part 2 opens, we learn that Nancy has held up pretty well over the years:
The firm yet placid mouth, the clear veracious glance of the brown eyes, speak now of a nature that has been tested and has kept its highest qualities; and even the consume, with its dainty neatness and purity, has more significance now the coquetries of youth can have nothing to do with it. (2.16.2)
At the same time, Nancy's rigidity is almost OCD: "the very pins on her pincushion were stuck in after a pattern from which she was careful to allow no aberration" (1.11.9). And she's definitely superstitious, so much so that that she'll give up an errand "if, on three successive times, rain, or some other cause of Heaven's sending, had formed an obstacle" (2.17.27).
Although she's a little more sophisticated, Nancy is as superstitious and, in her own way, as resistant to change as Silas. In fact, like Silas, she doesn't change much over the course of the novel. Godfrey grows up, but Nancy just gets older.