Because we see Mattie only though Ethan's (and the narrator's) eyes, and only hear her thoughts if she speaks them, she remains a rather mysterious character. Let's dig a little deeper into Mattie's character and see what we can learn.
Remember how Ethan hates to see anyone or anything treated meanly? This plays a part in his attraction to Mattie. People are mean to her and mistreat her. Her own family mistreats her at the time when she needs support the most. Twenty-year-old Mattie just lost both of her parents. All they left her was 50 dollars.
So why all the animosity toward this young lady? Apparently Mattie's father, who found wealth and power, had stolen from the very relatives on whom Mattie is now dependent. They want to punish Mattie for the fact that her father became wealthy, and doubly so for stealing from them to do it. They send her to take care of Zeena Frome, her ill cousin.
Based on her family's wealth, we can assume that Mattie was raised with advantages that Zeena and Ethan didn't have, though it doesn't seem that she has any education (according to Ethan, learning accounting almost kills her). She was most likely raised to be a happy-go-lucky young lady and eventually a wife. Yet, she's not spoiled and doesn't shy away from hard work. She doesn't see herself as superior to Ethan and Zeena because of her wealthier background.
Mattie changed from pampered daughter to a young woman dependent on a hostile world for survival. But, while Mattie's external circumstances have changed for the worse, she seems to be the same kind and unspoiled person she was when her parents were alive. Her changed circumstances don't seem to have hardened her heart – until after the accident.
Then again, some readers and critics see Mattie as frivolous and immature. In characteristic Shmoop fashion, we both agree and disagree. First of all, she's only about twenty-one, has little or no education, and very limited experience. It makes sense that she's a bit frivolous and immature. She wants to have fun, to frolic and play, and to have romance – though only, it seems, with Ethan. But what's your opinion?
As a young woman, Mattie seems to be truly in love with Ethan. Everything from turning down Denis Eady to declaring her love indicates that her affection for Ethan is sincere. If she simply wanted an easy way out of poverty, she could have married Denis Eady and been mistress to the Eady grocery fortune.
What we don't know is how Mattie feels about Ethan after the accident. That is, did she fall out of love with Ethan, or does she continue to love him even at the story's close? That we can't answer this question is part of what gives Ethan Frome its sense of tragedy. If Ethan and Mattie are still in love and still living with Zeena, then it sounds like the situation is truly awful. If they are no longer in love, that's perhaps even worse.
It's not Ethan but Mattie who initially suggests the suicide. Did this surprise you? This moment opens up her character. Just before, we learn that Mattie had fantasized about running away with Ethan some eight or nine months ago. Until now, Ethan (and us, the reader) hasn't know how she really feels about him. Now we understand that Mattie really wants to be with Ethan. Furthermore, she'd rather die than be without him. Since we can't track Mattie's inner movement to this point, we can't say whether the idea of suicide came upon her all the sudden, or if she had been considering it all along.
We know that Mattie was afraid Zeena was suspicious, and we know Mattie doesn't have anywhere else to go. She was probably way ahead of Ethan in seeing the hopelessness of their situation. That is, she probably already predicted that Ethan wouldn't or couldn't run away with her. Being a single woman with no money and virtually no education would be rough going. This story seems to be showing us how limited the options were for women in the Victorian period.
When we meet Mattie in the Epilogue, we see that her physical transformation is even more drastic than Ethan's. Mattie would only be about 45 years old at the end of the book, but is described as having not only aged drastically, but also of having "soured." According to Ruth, nobody was "sweeter" than Mattie before the accident. While the accident physically injured Ethan, and the events injured his heart and soul, he is still the sweet, kind, gentle man with whom Mattie Silver fell in love.
Mattie, however, is completely changed. Whereas she used to be the complete opposite of Zeena (see "Character Roles" and "Character Clues"), she has become almost exactly like Zeena. This transformation is foreshadowed in the scene where Mattie and Ethan have their night alone. Ethan suggests that Mattie sit in Zeena's chair, so he can see her better, but it doesn't work out quite the way he planned:
Ethan had a momentary shock. It was almost as if the other face, the face of the superseded woman, had obliterated that of the intruder. (5.3)
In other words, Ethan sees the face of Zeena ("the face of the superseded") remove from existence the face of Mattie ("the intruder"). Like most of the foreshadowing in this book, it only partially comes true. Mattie becomes prematurely aged, chronically ill, and chronically faultfinding, very much like Zeena was for much of her marriage to Ethan. But there is one big difference. Zeena was mostly silent. That's what the biggest problem was for Ethan. Transformed Mattie, on the other hand, is described as "droning querulously" (Prologue.62). In other words, she talks on and on and on in a grumpy, complaining, and argumentative way. So, like all the foreshadowing of the death, the foreshadowing of Mattie turning into Zeena only partially comes true.