The play is named after her, so you were expecting her to be the protagonist, right? Well, not so fast—as in the play itself, Major B gets kind of pushed aside while her father's ideals and beliefs take center stage. In fact, her father's reentry into her life manages to turn her life way upside down way quickly. But let's start at the beginning and work our way toward that . . .
When we first meet her, Barbara is super confident that her work with the Salvation Army can really do some good among the poor sinners of the world. In that way, she seems super idealistic…however, we're not sure she thinks of herself that way. Rather, she seems to see herself as a realist as far as human nature goes.
When her dad asks her if there's such a thing as a good man, for example, she replies:
No. Not one. There are neither good men nor scoundrels: there are just children of one Father; and the sooner they stop calling one another names the better. You needn't talk to me: I know them. I've had scores of them through my hands: scoundrels, criminals, infidels, philanthropists, missionaries, county councillors, all sorts. They're all just the same sort of sinner; and there's the same salvation ready for them all. (1.223)
So, okay, so admitting that everyone is a big fat sinner isn't really the mark of a pie-in-the-sky idealist—but her intense confidence that everyone can be saved? Well, some might call that faith, but we think idealism is a better word.
In any case, she's very cheerful and matter-of-fact even while pushing her beliefs. She offers them up enthusiastically and forcefully, but she doesn't seem to take herself overly seriously—and she isn't easily discouraged when others disagree with or outright contradict her.
For example, when a jerkwad named Bill is giving her a hard time at the Army, she refuses to let him get a rise out of her. When he is blustering on about not being afraid of Barbara after assaulting one of the other Army employees, she is smooth and cool as a hockey rink:
How could you be, since you're not afraid of God? You're a brave man, Mr. Walker. It takes some pluck to do our work here; but none of us dare lift our hand against a girl like that, for fear of her father in heaven. (2.106)
You can really tell a lot about Barbara and how she operates from this brief snippet of their interaction: She refuses to get outwardly confrontational or scolding, dressing up her criticism (and reminders about religion) as a compliment to Bill on his bad behavior. She actually almost brings Bill to tears with her placid and outwardly nonjudgmental comments on his violent behavior.
She keeps up the same pleasant banter even while trying to ease Bill toward conversion, pretending to applaud his efforts to resist religion even while pushing him toward it:
That's right, Bill. Hold out against it. Put out your strength. Don't let's get you cheap. Todger Fairmile said he wrestled for three nights against his salvation harder than he ever wrestled with the Jap at the music hall. He gave in to the Jap when his arm was going to break. But he didn't give in to his salvation until his heart was going to break. Perhaps you'll escape that. You haven't any heart, have you? (2.193)
Barbara's cheerful acceptance of Bill's jerkiness repeatedly knocks Bill off balance, and as a result she actually gets him pretty close to conversion . . . until her father breaks the spell.
Barbara's enthusiasm for the Army takes a hit when Mrs. Baines, the Commissioner at the Salvation Army, decides to accept huge donations from Barbara's father and a whiskey distiller named Bodger, even though these dudes' companies run counter to the Army's mission.
Mrs. Baines is blasé about the conflict of interest, but Barbara is truly appalled:
Why not! Do you know what my father is? Have you forgotten that Lord Saxmundham is Bodger the whisky man? Do you remember how we implored the County Council to stop him from writing Bodger's Whisky in letters of fire against the sky; so that the poor drink-ruined creatures on the Embankment could not wake up from their snatches of sleep without being reminded of their deadly thirst by that wicked sky sign? Do you know that the worst thing I have had to fight here is not the devil, but Bodger, Bodger, Bodger, with his whisky, his distilleries, and his tied houses? Are you going to make our shelter another tied house for him, and ask me to keep it? (2.413)
Mrs. Baines tries to pass the donation off as an opportunity to save Bodger's soul, but Barbara isn't having it:
I know he has a soul to be saved. Let him come down here; and I'll do my best to help him to his salvation. But he wants to send his cheque down to buy us, and go on being as wicked as ever. (2.416)
Unfortunately, Barbara seems to be in the minority among her coworkers (including her fiancé), and the donation goes through. Barbara is crushed, relinquishing her badge to her father and staying home from the army the next day.
However, somewhat incredibly, she ends the play reconciling an involvement with her dad's business with the Army's mission and work. She describes the turnabout in her thinking to Dolly toward the end of the play, after he has agreed to become the heir to the Undershaft business:
I was happy in the Salvation Army for a moment. I escaped from the world into a paradise of enthusiasm and prayer and soul saving; but the moment our money ran short, it all came back to Bodger: it was he who saved our people: he, and the Prince of Darkness, my papa. Undershaft and Bodger: their hands stretch everywhere: when we feed a starving fellow creature, it is with their bread, because there is no other bread; when we tend the sick, it is in the hospitals they endow; if we turn from the churches they build, we must kneel on the stones of the streets they pave. As long as that lasts, there is no getting away from them. Turning our backs on Bodger and Undershaft is turning our backs on life. (3.416)
So, while she still has her faith and her enthusiasm for spreading the word of God, she now seems a lot more reconciled to her father's view of the world, in which the righteous have to grab for power and money to ensure their good deeds can compete with…well, worse ones.
By the end of the play, Barbara's pluck and enthusiasm are back in full force, as she prepares for the new challenge of saving souls that aren't driven toward the Army purely by desperation and hunger. While talking to Dolly, she waxes enthusiastic about her dad's business and the surrounding village:
After all, my dear old mother has more sense than any of you. I felt like her when I saw this place—felt that I must have it—that never, never, never could I let it go; only she thought it was the houses and the kitchen ranges and the linen and china, when it was really all the human souls to be saved: not weak souls in starved bodies, sobbing with gratitude for a scrap of bread and treacle, but fullfed, quarrelsome, snobbish, uppish creatures, all standing on their little rights and dignities, and thinking that my father ought to be greatly obliged to them for making so much money for him—and so he ought. That is where salvation is really wanted. My father shall never throw it in my teeth again that my converts were bribed with bread. [She is transfigured]. I have got rid of the bribe of bread. I have got rid of the bribe of heaven. Let God's work be done for its own sake: the work he had to create us to do because it cannot be done except by living men and women. When I die, let him be in my debt, not I in his; and let me forgive him as becomes a woman of my rank. (3.422)
Barbara's faith and enthusiasm remain high, but she's become a lot less idealistic about the way the world works. You could argue that she's become cynical, but her cheerfulness and still-intense idealism at the end kind of works against that reading. She's still Major Barbara—she just has a deeper awareness of being on the Undershaft payroll…like basically everyone else in the world, apparently.