Major Barbara, as Barbara Undershaft is known at her post at the Salvation Army, is a young woman from an upper-class family who feels called to save souls rather than enjoy a life of leisure. Confident that God offers salvation to every repentant heart and that she can make a difference in the lives of poor people, Barbara works tirelessly at her job. Barbara becomes disillusioned with the Salvation Army when she witnesses her leaders accept a donation from a local distiller as well as from her father, a munitions manufacturer. Having resigned her post because of what she views as the organization's hypocrisy, Barbara later comes to believe that good and evil are not as distinct as she once thought and that she may need to be more pragmatic to help bring about real change in society.
Born out of wedlock, Andrew Undershaft has made his millions by inheriting a munitions company passed on solely to foundlings like him. Undershaft is an amoral, pragmatic man who believes the greatest evil in the world is poverty. He has no qualms about making money from instruments of death. He believes money is the highest good. Impressed with the intelligence, loyalty, and hidden brashness of his future son-in-law, Undershaft feels contempt for his own principled and unfocused son. He is ambivalent toward his wife and unrelenting in his resolution to keep his son from following in the business. Further, Undershaft loves his daughter and wishes to convert her to his way of thinking. He manipulates her by donating money to and thus revealing the hypocrisy of the organization into which she has put her faith. Eventually, he is able to convince her his money has greatly benefited his employees.
Adolphus Cusins is an educated, liberal young man and a scholar of ancient Greek. His love for his fiancé Barbara makes him happy enough to aid in her religious pursuits, though he himself is not a true convert. While he finds Undershaft manipulative and morally reprehensible, he can't help but like him. Although his parents are legally wed in Australia, their marriage is not recognized in England. The law thus makes him a foundling and eligible to take over Undershaft's business. He wants to help the poor and pledges to use the business to help end war.
Lady Britomart is a domineering mother who separated from her husband because he refused to conform to her ideas of morality and because he was reluctant to make their son Stephen heir to the business. She wants to ensure her children have enough money to continue their comfortable lifestyle. Although she objects to the source of her husband's fortune, she accepts their dependence upon it. What enrages her most is her husband's insistence upon disinheriting his own son, which Lady Britomart is convinced is morally wrong.