The first thing that strikes Martin Arrowsmith about Leora's character is her laid-back way of approaching people, even those who are higher up the social food chain. This casual attitude infuriates Martin at first—he's a young medical student who deman ds respect and she's a beginning nurse. As the novel tells us,
Her indolent amusement, her manner of treating him as though they were a pair of children making tongues at each other in a railroad station, was infuriating to the earnest young assistant of Professor Gottlieb. (6.3.10)
But at the same time, Martin recognizes that Leora has everything that is lacking in his fiancé, Madeline Fox. Madeline is a swanky upper class girl who is all about acting prim and proper all the time. Leora, on the other hand, only cares about whether or not someone is a good person, and couldn't care less about how much money they have. This kind of attitude is much more in line with Martin's true values.
The one thing that Leora is not casual about is keeping Martin all to herself. As she tells him in a passionately jealous moment, "I'm a cavewoman, and you'd better learn it, and as for that Orchid, with her simper and her stroking your arm and her great big absurd feet--- Orchid!" (19.4.39). Leora knows that Martin sometimes hits on younger women and she won't stand for it.
As the book unfolds, it becomes pretty clear that Leora often understands Martin Arrowsmith better than he understands himself. Time and time again, Martin hears the siren song of money and power, and Leora has to step up and remind him that these aren't his true values. As she says at one point, "Are you going on for the rest of your life, stumbling into respectability and having to be dug out again? Will you never learn you're a barbarian?" (20.3.42). This is especially amazing because you'd expect Leora to want Martin to climb up in the world. But the truth is she wants just the opposite. She wants him to stay free and true to himself.
When she's not telling Martin to follow his dreams, Leora tends to sit around by herself. But she claims that she never gets bored or lonely while Martin spends his days and evenings in his laboratory. Her entire life seems to revolve around supporting him. As the narrator tells us, "She sat quiet (a frail child, only up to one's shoulder, not nine minutes older than marriage, nine years before), or she napped inoffensively, in the long living-room of their flat, while he worked over his dreary digit-infested books till one" (27.5.22).
It's very hard to say whether Leora is being totally honest when she says she's fine being alone for most of her life. But never in this book will she admit to Martin that she wants anything other than for him to be happy. The one thing she ever lets slip is that she wants the two of them to go to France some day. It's pretty crushing that she dies before this can ever happen.
We don't get a whole lot insight into Leora's private life in this book, since she spends most of her energy supporting Martin and his ambitions. In a rare moment, though, Leora reveals that some day she'd like to travel with Martin to France: "Sandy, the one thing I want to do, maybe ten years from now, is to see Touraine and Normandy and Carcassonne. Could we, do you think?" (22.4.7). Of course Martin is more than happy to say yes, since he has always wanted to make Leora happy but has never gotten her to admit to wanting something.
By the end of the book, Leora still hasn't fulfilled her one wish of going to France with Martin. And the brutal truth is that she dies of plague before she can ever realize this dream. If that weren't enough, Martin is off kissing some other woman while Leora dies, frightened and alone. The book gives us a horrible look into her final moments as she shouts: "You will come! I know you'll come and help me! I know. You'll come! Martin! Sandy! Sandy!" (35.4.20). This is without doubt the hardest passage to read in the entire book, and it gives us one last look into the mind of poor Leora.