We know that the narrator is entirely biased and that he portrays everything the way he imagines Ethan must have seen it (see "Narrator Point of View" for more specifics). Though logically we know that Zeena is a victim – her husband has fallen in love with another woman and constantly considers leaving her – she comes off as the bad guy. She appears to be a mean, grumpy woman who browbeats her husband, holds him back from all his hopes and dreams, separates him from the woman he loves, and abuses and abandons her relative, Mattie Silver.
All this is pretty much true, but we can't blame Zeena for wanting Mattie to leave the house and fast – Zeena's suspicions are entirely correct.
Despite what Zeena actually does or says, the way she's described makes certain that we view her in a negative light. She has an unpleasant physical appearance, having "grayish tinge[d]" skin, false teeth, and a "puckered throat." The light of a candle reveals the "microscopic cruelty and fretful lines of her face." Even her voice is described as an obnoxious "flat whine." Because the narrator is loyal to Ethan's biased point of view, a sense of sympathy for Zeena does not leak out until the end of the pickle dish scene (which we explore more thoroughly under "Symbols, Imagery, Allegory"). There we understand that Zeena does love Ethan (though she doesn't know how to show it) and does want to be his wife. At this moment we can see clearly just how broken her dreams are, just how badly things have gone for her.
Zeena seems to flip-flop between the roles of nurse and patient. In the early 1900s medicine was much less of a developed science than it is today. (If you are curious about the kinds of medicines Zeena was buying, click here.) Interestingly, the story goes to some trouble to convince us that Zeena's illness is in some way imaginary or psychological. The biggest piece of evidence for this comes from Ruth, who is actually defending Zeena to the narrator.
"Zeena's done for [Mattie], and done for Ethan, as good as she could. It was a miracle, considering how sick she was – but she seemed to be raised right up just when the call came to her. Not as she's ever given up doctoring, and she's had sick spells right along; but she's had the strength given her to care for those two for over twenty years, and before the accident came she thought she couldn't even care for herself." (Epilogue.23)
We know that when Zeena became Ethan's wife, she transformed into a sick woman. According to Ethan, she learned how to care for the sick by being sick herself. Yet, when Mattie and Ethan are injured, she miraculously becomes relatively healthy again, and remains so for at least the next 24 years. (This also means that the medicine the narrator sees Ethan picking up from the post office in the Prologue might not be for Zeena.) This suggests that she can only be healthy when she is doing what she loves to do, which is caring for the sick.
That's the charitable explanation, and one with which Edith Wharton might also take issue. Her portrait of Zeena seems very much designed to criticize those who use illness as a tool for manipulating others. Zeena appears to switch back and forth between illness and health depending on which gives her more power in a given situation, and Wharton's contempt for these kinds of maneuvers shows through.
But Zeena has some physical signs of illness that we can't ignore. Perhaps depression contributed to her premature aging, but could it really account for her lost teeth? Dental care was probably scarce in her neck of the woods, but if she lost all her teeth before she turned forty then her health problems couldn't be all in her head.
One thing this novella gets at through the character of Zeena is that both the body and the mind are mysteries, and we will probably never tire of trying to figure out how they work, together and separately.
Zeena seems to be the character that changes the least – from beginning to end she is healthy when she has sick people to care for, and unhealthy when she doesn't. We also get the idea that from beginning to end she doesn't have the best bedside manner. The question is why did she decide to take care of Mattie and Ethan? Was it out of sympathy, love, vengeance, shame, or duty? Or was it for some other reason?