Who can defined as a "flat" character in A Visit from the Goon Squad?

A flat character, by definition, is two-dimensional and fails to change over the course of a text. This is typically tied to another characterization called "static." A static character is identified as one who does not change—thus, all flat characters are static. By contrast, a "round" character is complex and three-dimensional. A round character will undergo a change; this is also associated with what is known as a "dynamic" character (i.e., a character who undergoes a change).
One flat character from Jennifer Egan's A Visit from the Goon Squad is Lou Kline. Lou produces music and lives the ultra-extravagant lifestyle that is typically associated with this profession. He spends money without thought, does drugs, and has numerous superficial relationships with women. He refuses to change his lifestyle and dies alone.
Lou is a flat character because he is a static, readily-identifiable stereotype, and almost all readers can correctly assume what will come of him in the end. His demise—the result of his lifestyle and a refusal to change—is inevitable. In order to not be defined as a flat character, Lou would have needed to come to terms with who he is, identify his short-comings, and/or change his life.