In “A Painful Case,” Mrs. Sinico is a foil to Duffy, meaning that her presence in the story reveals important information about Duffy, the protagonist. While both characters start the story feeling alienated from other people—Mrs. Sinico is trapped in a loveless marriage, and Duffy lives with almost no human interaction—Mrs. Sinico is far more in touch with her needs, desires, and emotions.
For example, after regularly meeting Duffy discretely to discuss their innermost desires and thoughts, Mrs. Sinico reaches out to touch his face, showing that she longs for physical closeness with him. Rather than affirming this human desire for connection, Duffy becomes “disillusioned” with her and internally lambasts her for sexualizing their relationship.
Their separate reactions to Duffy’s decision to ultimately end their friendship captures Mrs. Sinico’s emotionality in comparison to Duffy’s coldness:
They agreed to break off their intercourse: every bond, [Duffy] said, is a bond to sorrow. When they came out of the Park they walked in silence toward the tram; but here [Mrs. Sinico] began to tremble so violently that, fearing another collapse on her part, he bade her good-bye quickly and left her.
The fact that Mrs. Sinico “tremble[s] so violently” that Duffy thinks she may collapse—and that Duffy decides, in this moment of vulnerability, to “quickly” say goodbye and leave her—demonstrates Duffy’s emotionally removed and callous nature. While Mrs. Sinico is the one who ends up losing herself in alcoholism and dying, Joyce hints that the emotional and sexual repression of people like Duffy (who exists in a dissociated state “at a little distance from his body”) is possibly an even worse fate.