Consider the setting, plot, and character in The Jade Peony. Which do you think is the most important in this story?

In my opinion, the setting is the most important of the factors you've provided. The time in which the story is placed directly affects the way the Japanese are viewed in the world and suggests the reasons for this treatment. Japan's occupation of China, as well of the bombing of Pearl Harbor, placed Japanese immigrants in a horrific position worldwide. Note, this doesn't make it right, but it is certainly an example of cause and effect.