A Codeception developer designs, develops, and implements automated tests for web applications using the Codeception testing framework. They work to ensure the applications' functionality, performance, and reliability by writing and executing various types of tests such as acceptance, functional, and unit tests. They also debug code, fix software defects, and integrate the testing framework with other tools for continuous integration and deployment. Additionally, they collaborate with other team members to understand testing requirements, improve test coverage, and maintain the testing documentation. Their ultimate goal is to enhance software quality and deliver a seamless user experience.