A Functionality Testing developer is responsible for examining each function of the software application to ensure it works as intended. They design and execute test cases, identify bugs, and report them for rectification. They verify the interaction of various software systems and ensure the software's compatibility with different environments. They also validate the software's usability, performance, and security. They play a crucial role in improving the quality of the software, ensuring it meets user requirements and is free from defects. Additionally, they collaborate with other team members to fix issues and continuously enhance the software's functionality.