Compared to the other powerhouse women in the novel, Thomasin is a bit dull. She's nice, she tries to be a good wife and mother, and she's sympathetic. True, she's not super nice to Diggory and she makes a big mistake in marrying Damon. But she's nowhere near the disaster that, say, Eustacia is.
However, Thomasin only appears bland because she's surrounded by over-the-top melodrama.
"I agreed to it," Thomasin answered firmly. "I am a practical woman now. I don't believe in hearts at all." (2.8.12)
Thomasin is a pragmatist. We can see this in her approach to everything, be it nature or relationships. And this trait definitely helps her stand out in a novel filled with drama queens and emotional meltdowns and throw-down arguments.
But this isn't to say that Thomasin is cold-hearted in any way. She's as emotional as everyone else here – in all the scenes leading up to her marriage, and in her marriage, she isn't very happy, but she puts up a strong front and makes the best of things. And it's this emotional balance that truly makes Thomasin stand out. While everyone else is running around sobbing and yelling and delivering dramatic monologues, Thomasin is calmly and quietly going about her life.
Thomasin definitely knows how to make the best of a bad situation. She pretty much just rolls with the punches and makes lemonade out of lemons, to use the cliché. We especially see this in her relationship with Damon.
"Now, I put it to you: would you at this present moment agree to be his wife if that had not happened to entangle you with him?"
Thomasin looked into the tree and appeared much disturbed. "Aunt," she said presently, "I have, I think, a right to refuse to answer that question."
"Yes, you have."
"You may think what you choose." (2.2.22-26)
Thomasin shows a lot of maturity here. Just imagine how this scene would have gone with Eustacia – she would've let loose a stream of explanations and regrets and insults. But Thomasin plays it cool.
However, Thomasin isn't entirely calm and rational. She's a normal person with normal emotions and she gets justifiably upset about her situation with Damon.
Thomasin lowered her face to the apples again. "I am a warning to other [...]" she said in a low voice. "What a class to belong to! Do I really belong to them? 'Tis absurd! Yet why, aunt, does everybody keep on making me think that I do, by the way they behave towards me? Why don't people judge me by my acts? [...] I wish all good women were as good as I!" (2.2.9)
Thomasin really demonstrates the novel's themes of gender and especially gender double-standards here. The scandal that derails Thomasin's life has virtually no effect on Damon. Thomasin is ostracized from the community and is subject to gossip and rumor.
It's also notable that, in both of these scenes, Thomasin is closely connected to nature. She looks at a "tree" and "apples" before responding to any question or voicing her thoughts. It's as though nature calms her and helps her to articulate her ideas.
These nature connections are a huge part of Thomasin's character. In contrast to Eustacia, who is always tied to the "heavens" and the sky, Thomasin is linked to the earth and the heath itself; she plays a sort of calming earth-mother role in the novel, something that's reinforced when she herself becomes a young mother.
Bird metaphors also surround Thomasin. During the awkward period following her failed elopement and prior to her marriage, Thomasin is effectively a caged bird. We can see how isolated she is at the Christmas party, where she has to hide upstairs out of embarrassment (2.6.41), and as she walks off to get married.
Like a caged bird, Thomasin isn't really free – she's stuck in an unhappy marriage and she's bound by social convention. But Thomasin makes the best of the situation by accepting it and using her practicality to get through difficult times. So, in a way, Thomasin achieves harmony with her surroundings by accepting them for what they are.
Given how cool Thomasin is – smart, fairly savvy, good mom, overall decent person – it makes sense that she gets a happy ending, right? Wrong. Originally, Hardy hadn't planned for her and Diggory to marry. Diggory was supposed to have remained a bohemian wanderer, and Thomasin was supposed to remain a single mom.
Though Hardy's original ending may seem mean, it's actually more fitting with Thomasin's character, if you think about it. Ever the pragmatist, Thomasin probably never expected a fairy tale ending. Thomasin's published ending sort of has her jumping ship from realism to romanticism. But Thomasin was never immune to love and romance, as we see with her relationship with Damon, so maybe a second chance at romance isn't totally off-the-wall for her. At any rate, Thomasin is probably the most balanced and well-rounded character in the book. She mixes her flaws with a lot of good qualities and, like the book itself, she's an interesting blend of romance and realism.