Hetty Sorrel is a very pretty girl, and an efficient-enough dairymaid. But she isn't particularly accomplished. Or particularly smart. Or particularly nice. You are perfectly within your rights to wonder what a stand-up dude like Adam sees in her.
Still, even if you don't think much of Hetty Sorrel as a person, do not underestimate her as a character. One of the triumphs of Adam Bede is its depiction of Hetty's "silly little imagination," as the narrator calls it (9.5). We are shown her vanities (and there are a lot of vanities to show) in remarkably precise detail. We get a bird's-eye view of the "earrings and the locket" in her "treasure drawer" (31.11).
We even come to understand how Hetty's grand ambition—marrying rich young Arthur Donnithorne—took shape. Even if that ambition is a lot of malarkey:
Reasons why he could not marry her had no existence for her mind; how could she believe in any misery that could come to her from the fulfillment of all she had been longing for and dreaming of? (31.8)
A more dismissive novelist would make us shake our heads and scoff at poor Hetty, but Eliot's brand of realism won't permit that. Just check out that quote—a more dismissive novelist would be pooh-poohing Hetty's fantasies of wealth. Eliot, instead, gives us the very questions that Hetty asks herself in this moment of crisis, without judgment.
Remember, this is a book about the "existence of insignificant people" and its "important consequences in the world" (5.63). Hetty isn't "significant," but Adam Bede treats her thoughts, emotions, and problems as important.
We're not going to sit here and try to convince you that Hetty is a great individualistic heroine, an Anna Karenina or a Madame Bovary. Truth be told, there are times when she's barely a cut above Gretchen Wieners from Mean Girls—her hair is absolutely full of secrets. But mean girls have feelings too.
And Hetty's feelings are on full display when her life takes a very non-Tina Fey comedy turn. When the pregnant Hetty is left alone to suffer, Eliot doesn't skimp on harsh realities. Hetty "must wander on and on, and wait for a lower depth of despair to give her courage" (37.44). Yeah, this is bad. But Hetty is aware of what she truly needs in life (courage) in a way that she tends not to be early on.
This self-awareness intensifies when Hetty gets locked in prison for child murder. Dinah coaxes a powerful confession out of her, sure. Hetty, however, examines her own guilt and her own agonized feelings in a bracingly honest way:
"I don't know what I felt till I saw the baby was gone. And when I'd put it there, I thought I should like somebody to find it and save it from dying; but when I saw it was gone, I was struck like a stone, with fear." (45.64)
Sometimes, maturity means not having the answers. Hetty faces her moments of ambivalence, her "I don't know" moments. What happened to the spoiled girl who just wanted to marry Arthur?
Hetty changes a lot through suffering. This is exemplified through Hetty's relationship to Dinah. Early on, Hetty is just happy that Dinah isn't "disapproving or reproachful" (14.10). Later, however, Hetty positively realizes that Dinah is a kind and virtuous person—and realizes the power of communication and confession. Weirdly enough, Hetty becomes a wiser and more considerate person by killing her child and going to prison.
Just don't try this at home, folks.