A Cobertura developer specializes in using the Cobertura tool, a free Java-based utility that calculates the percentage of code accessed by tests. They design, write, and execute test cases to ensure code quality and reliability. Their responsibilities also include identifying and fixing software defects, improving code coverage, and integrating the Cobertura tool into build processes. They analyze the Cobertura reports to understand the parts of the code not covered by tests and then create new tests to cover those areas. They work closely with other developers to ensure that the application is free of bugs and performs effectively.