A Jade.js developer designs and builds web applications using the Jade.js template engine, now known as Pug. They write server-side logic, define data structures, implement security and data protection measures, and ensure high performance of applications. They also integrate front-end elements built by other team members into the application, ensuring both server and client sides work together smoothly. They manage API resources and handle user interactions on the web pages. Their role includes debugging applications, updating existing features, and collaborating with other team members to optimize the development process. They also ensure code quality and maintainability.