A Koa.js developer is responsible for designing and implementing web applications using Koa.js, a next-generation web framework for Node.js. They write server-side logic, define and maintain databases, ensure high performance and responsiveness, and create secure back-end features. They work closely with front-end developers to integrate user-facing elements and ensure the server-side web logic is functioning properly. Koa.js developers also conduct testing, troubleshoot issues, and maintain code quality. Additionally, they may need to implement data protection measures and work on improving the application's performance. They must have strong coding skills, problem-solving abilities, and understanding of asynchronous programming.