What is the difference between flat and round characters? Round characters are one-dimensional characters with less important story lines; flat characters are multifaceted with story lines that are essential to the plot. Flat characters are one-dimensional characters with less important story lines; round characters are multifaceted with story lines that are essential to the plot. Flat characters usually have complicated story lines so that they can become the hero of the plot. Round characters, like the servant, often have minor story lines.