Fitzgerald's The Great Gatsby is full of flat characters--people who are one-dimensional and about which we know little other than their social status and their bad behavior. It is an interesting area of exploration to think about Jay Gatsby as a flat character, which in one sense he is. He only shows or tells most people what he wants them to see or know about him. All most people see of him is his beautiful mansion and his spectacular parties, at which they can only speculate about the rest of his life. To most people, then, he is a flat character.
To the reader who knows Gatsby through Nick, though, he must be considered a round character. We know his past and we therefore understand what motivates him. We see him outside of his elusive role as party host and understand his yearnings and longing for a life with Daisy, the woman he loves and for whom he has transformed himself. When Gatsby dies, the reader mourns the loss of a romantic soul to such a tragic death. Gatsby is a round character.