A GraphQL and Relay developer designs and implements APIs using GraphQL, a query language for APIs, and Relay, a JavaScript framework for building data-driven React applications. They write efficient and performance-driven code to fetch data, manipulate and update the database. They also optimize applications for maximum speed and scalability, ensuring system consistency and validation. The developer provides solutions for managing application state and fetching data efficiently with the use of GraphQL and Relay. They also maintain, troubleshoot, and improve existing systems, and collaborate with other team members to ensure seamless integration.