Who is the antagonist in the short-story, Love Letter?

J. is the antagonist in the story, she symbolizes conflict. The author uses her character as a primary source of tension throughout the narrative. Because Robbie has a deep affection for her, he is reluctant not to get involved in her profound troubles. Her particular conflict creates internal conflict for Robbie, and thus for the narrator. Yet this issue proves nearly impossible to solve, and does not possess a singular answer or resolution.