Ethan is characterized by the contrast between his actual occupation and the job he wishes he had. He wants to be an engineer, to study science, and cultivate a deeper understanding of nature. He has no desire to do farm work, and even less to run a sawmill. The fact that he seems stuck in his occupation, with no chance for escape, defines a crucial aspect of his repressed character. When we think about the fact that the narrator is also an engineer, the two become like strange shadow versions of each other, one successful, and one a failure; one free, and the other trapped. The narrator is everything Ethan wishes to be, professionally speaking, and Ethan shows narrator how his own life could have turned out differently. (See "Narrator Point of View" for more.)
Ethan, Zeena, and Mattie are heavily characterized by their family life. Their strange life together is uncomfortable and dangerous in the early days, and downright disturbing in the later ones. What began as a love triangle becomes a lifetime arrangement. Ruth makes it clear to the narrator that everybody in town is appalled by the Fromes' living situation, and the fact that the narrator is the first overnight guest at the Frome house in twenty years suggests that shame pervades their home life.
The narrator is initially fascinated by Ethan's physical appearance. Ethan is obviously a powerful man stricken with a terrible disfigurement. Before his injury Ethan was characterized by his height and his bulk as a strong, reliable person. Zeena's physical appearance says a lot about her character. Her sickly and unpleasant appearance combines with the descriptions of her cold, mean behavior to create the impression of a thoroughly unpleasant person, inside and out.
Mattie on the other hand is characterized by the color red. As we discuss in "Symbols, Imagery, Allegory," the color red has many implications, including seduction, sin, and passion, all of which contrast intensely with Zeena's cold personality. The lively color red also seems to match Mattie's general disposition. She is friendly, pleasant, always either dancing or doing some kind of work.
But most of all, young Mattie is characterized by her long eyelashes. They almost have a mind of their own. Not only can they impart a "fugitive warning," but they can also practically pull Mattie up the stairs (2.80). Long eyelashes have long been as sign of greater than average beauty (for both males and females). They are also symbols of temptation. By the end of the story, eyelashes are probably the least striking thing about Mattie. The most striking things is that she looks very old and appears to be almost completely paralyzed. Ironically, Zeena seems to have regained her strength and health.