An Isotope.js developer is responsible for creating dynamic and interactive layouts on web pages using the Isotope.js library. They utilize the library's filtering and sorting capabilities to manage, arrange and design elements on a webpage. They manipulate data and elements to create a seamless and intuitive user interface. Moreover, they work with other JavaScript libraries and frameworks to ensure compatibility and optimal performance. They also debug and optimize Isotope.js code to improve website functionality. Additionally, they might be involved in maintaining and updating the library, and providing technical support to other developers using Isotope.js.