A Marionette.js developer specializes in using the Marionette.js framework, a composite application library for Backbone.js, to build complex JavaScript applications. Their role involves creating, testing, and maintaining web applications, ensuring optimal performance and responsiveness. They design user-friendly interfaces, write reusable code, and manage data structures and models. They also handle events, views, and behaviors of the application. Additionally, they troubleshoot and resolve issues, collaborate with other team members, and provide technical guidance. Their work enhances user experience, improves web functionality, and contributes to the overall success of web development projects.