Catherine Barkley is a static character in the novel; that is, she does not undergo any major transformation over the course of A Farewell to Arms. Apparently she has done her growing and changing before the story began. Hemingway can therefore "use" Catherine as a foil to Henry and an index of his maturation. She is like a constant in a scientific experiment. Of course, this does not make her any less interesting than Henry, and it certainly makes her no less admirable. She's simply less dynamic.
The writer's use of Catherine to contrast dramatically with Henry — to show us just how much learning and growing he has yet to do — begins in the first scene they share together. Henry is still playing childish games: telling her he loves her when he doesn't, for instance. Soon, however, the tables are turned. Catherine not only resists Henry's advances; she reveals that she knows he has been playing a game. Apparently she has been playing one too: "You don't have to pretend you love me," she tells Henry. "You see I'm not mad . . . " Here Catherine proves wiser than she at first appeared — wiser in the ways of the world so far than the easily deceived Henry. Indeed, the latter may be attracted to Catherine precisely because of her aura of hard-earned maturity. Well, that and her hair.
Catherine rejects organized faith, and yet (unlike the priest-baiting officers at the front) she is no nihilist. She lives by a definite, unshakeable value system, and what she values is love. During one of the many nights they spend together in Milan, the couple discusses marriage, which Henry wants but Catherine resists for practical reasons. It would necessitate their separation, she explains — more worldly than he, despite his battlefield experience. She reminds him and us of her having been formally engaged to the soldier who died. Then Catherine tells Henry that she has no religion. She quickly corrects this statement, however, explaining "You're my religion."
Catherine also tells the admitting nurse at the hospital where she goes to give birth at book's end that she lacks a formal religious affiliation of any kind. Henry too calls himself an agnostic, and yet, as virtually anyone would, Henry tries bargaining with God in his desperation at Catherine's impending death. Catherine, on the other hand, retains the courage of her convictions. To the very end, Catherine remains the somewhat stronger of the two. "Just you," she requests of Henry in response to his offer of a priest's visit. Despite everything, love is her religion until the instant she dies.
For much of the novel, Catherine is also more developed than Henry as a Hemingway hero, modest and truthful. Note that while Henry tolerates the "professional hero" Ettore Moretti, Catherine dislikes him intensely. "We have heroes too," Catherine says of Moretti, "But usually, darling, they're much quieter." Additionally, Catherine is distressed by the rigged racetrack betting in which Meyers is involved. "I don't like this crooked racing!" she declares. She suggests to Henry that they bet on a horse they've never heard of, and although it finishes fifth, she feels "so much cleaner." Again, while Henry is tolerant of a certain amount of corruption, Catherine demands purity.
The notion of Catherine's special bravery — another of her heroic qualities — is also introduced during the Milan idyll. With characteristic modesty, she suggests she would like to be brave. When Henry naively suggests that "Nothing ever happens to the brave," the more-experienced Catherine counters with the statement, "They die, of course." And Catherine's extraordinary fortitude is very much in evidence during the escape across Lake Maggiore. Despite her fairly advanced pregnancy, she not only travels through the November night in an open boat but also offers to hold the umbrella so it will serve as a sail. She steers and bails and even rows for a while, always maintaining a sense of humor.
Significantly, we don't doubt Catherine's bravery and stoicism as she perishes; we have been prepared for it by scene after scene in which she displayed just these qualities. What does surprise is her statement, "It's just a dirty trick," which seems to ally her with the cynical, nihilistic officers in Henry's unit. Perhaps Catherine has changed over the course of the novel after all.