Shaw's Major Barbara is a play of social satire that abounds in situational irony, using surprising plot twists and outcomes to make serious points about how conventional thinking fails to address major social problems and is blindfolded by a desire for conformity.
Among the strongest characters in the play are Undershaft, Cusins, Lady Britomart, and Barbara. Undershaft and Cusins both are foundlings and social outsiders, Undershaft in his unconventional attitude and work manufacturing weapons, and Cusins in his dedication to classical philology. Barbara is a leader in the Salvation Army, one of the few religious organizations of the period which had full gender equality. Even if she eventually proves ideologically misguided, she is strong and independent by nature and wants more from life than simply marrying a traditional wealthy husband and running a household.
Stephen, on the other hand, is portrayed as a conventional upper-class male within a patriarchal society, who, despite his sense of entitlement, lacks any ability to think independently, run a business, or even show genuine leadership or obtain respect. The key reason that Undershaft must choose a foundling as a successor is precisely because foundlings must be strong individuals who have made their own way in the world and developed strength of character in adversity, while heirs such as Stephen, who are brought up in luxury and never needed to struggle, are inherently weak. While Stephen displays a great reverence for convention and horror at manufacturing of weapons, Undershaft believes that the worst scourge of humanity is poverty and that his work in providing jobs saves more people than churches and charitable organizations.
Perhaps the major difference between the characters, though, is that Undershaft is honest and outspoken, saying what he thinks untrammeled by what he thinks should be said, while Stephen is a hypocrite who is overly concerned with the image he presents and how other people think of him.