An Axios.js developer is responsible for making HTTP requests to REST endpoints to access or manipulate data using the Axios library in JavaScript. Their role includes handling requests and responses, error handling, and ensuring data integrity. They manage asynchronous operations, intercept request and response, transform request and response data, and cancel requests. They work on both frontend and backend frameworks like React.js or Node.js. In addition, they also ensure the smooth integration of the Axios library into the project, and improve the application's performance by optimizing the code. They also implement security measures to protect sensitive data.