A Hapi.js developer is responsible for creating server-side application logic using the Hapi.js framework for Node.js. They design and implement data models, connect applications with other web services, and support the front-end developers by integrating their work with the Hapi.js application. They also ensure high performance and responsiveness of requests from the front-end. Additionally, the Hapi.js developer performs API integrations, troubleshoots and resolves bugs, maintains code quality, organizes the server-side logic, and ensures high security and data protection. They are often involved in the entire web development process from concept to final product.