An FXML developer is primarily responsible for designing and building user interfaces for JavaFX applications. FXML is a scriptable, XML-based markup language that allows developers to define the user interface of a JavaFX application separately from its behavior. The developer uses FXML to structure and organize the graphical user interface, making it easier to visualize, modify, and maintain. They work closely with Java developers to ensure seamless integration between the user interface and the application logic. Additionally, they may also be involved in debugging, testing, and resolving issues related to the user interface of the JavaFX application.