An FxCop developer is primarily responsible for using the FxCop tool, a code analysis tool provided by Microsoft that helps to enforce coding standards and best practices in .NET applications. The developer uses FxCop to analyze the source code or compiled assemblies of an application, checking for conformance to Microsoft's .NET Framework Design Guidelines. This includes verifying naming conventions, library design, localization, and performance. The developer also addresses issues flagged by FxCop, refining the code for maximum efficiency and readability. This role requires a strong understanding of .NET framework and coding standards.