A Class Testing developer is responsible for verifying the functionality and reliability of individual classes in object-oriented programming. They create and execute test cases to ensure that each class, as a standalone entity, works as expected. This involves checking data handling, interactions, and interfaces of the class. The developer also validates the methods and functions within the class, ensuring they perform their tasks correctly. They also look for potential errors or bugs, and debug them. The goal is to ensure each class can integrate smoothly with others, thereby contributing to the overall functionality of the software system.