Cast as the poet within the play's triumvirate of madmen, Cusins is a "slight, thin haired, and sweet voiced" Greek scholar determined to marry Barbara at all costs. As Shaw notes, it is not love that drives him to Barbara but an unmerciful "instinct." He joins the Salvation Army as a result but, as Undershaft remarks, the hollowness of his Army drum symbolizes the hollowness of his conviction in the organization. Indeed, Cusins uses the Army much like he does his drum, considering it an instrument for his Dionysian ecstasies. Understanding himself as a "collector of religions," he does not subscribe to any particular "pathfinder of salvation," but seeks only that which brings reality, power, and jubilation to the people. A student of the great democrat Euripides, he clings above all to a belief in this love for the people.
Upon Undershaft's attempt to convert him to his gospel, Cusins is particularly seduced by what he describes as his Mephistophelean "ecstasy of mischief." By "mischief" Cusins refers to his calculated sense of irony and wily argumentation in securing demonstrating to Barbara how he holds the Salvation Army and all its guests in his power. He alone perceives this irony as Undershaft adopts him as a sort of coeval in the previous scene.
This demonstration gives Cusins over to Undershaft's cause. As he cries joyfully, the spirit of "Dionysus-Undershaft" possesses him entirely. Ultimately Cusins will become his successor, shedding his name to assume that of the firm. Unlike Andrew, however, Cusins takes over the armory in the name of his love for the people. He declares that through the armory, he will abandon his bookish studies, work that only served to arm the intellectual against the commoner, and make power for the masses. For Cusins, gunpowder is a power accessible to the common people and that forces the "intellectual oligarchy" to exert itself for the general good.
Shaw describes Cusins as a "determined, tenacious, intolerant person" who presents himself as he is: "considerate, gentle, explanatory, even mild and apologetic, capable possibly of murder, but not of cruelty or coarseness." The "lifelong struggle of a benevolent temperament and high conscience against impulses of inhuman ridicule" has wrecked his constitution.