This play, The Good Person of Szechwan (aka The Good Woman of Setzuan), makes an ideological point about what happens to good people in a world where goodness is not rewarded and, seemingly, cannot last.
Shen Te is split in two, becoming both herself and Shui Ta. The two characters are literally doubles. They are the same person. Shui Ta is created by Shen Te when she puts on men's clothes and a mask.
Each of these characters is a persona of the same person, Shen Te. One persona is generous and good. The other is prudent, shrewd and business-like.
Shui Ta is the male persona Shen Teh takes on when she needs to be a tough businessman.
Shen Te, the good young woman, finds that her acts of generosity lead her to be taken advantage of to the point where she will lose her ability do any more good deeds. The shop that allowed her to give shelter to her cousins will be taken away from her and she will not be able to afford the rice that she gives away to the hungry people of the town. All this is a direct result of Shen Te's attempts to act out of goodness and generosity.
To save her shop and herself, Shen Te dons the disguise of Shui Ta and deals shrewdly with the people who are taking advantage of her. Shui Ta represents the necessity that Shen Te feels regarding self-defense. Being good and only good, she would not have lasted long.