A Behat developer is responsible for automating tests for web applications to ensure functionality and performance using the Behat testing framework, a PHP-based tool. They write and implement test scenarios in Gherkin language, a human-readable language that describes business behavior without detailing how the software is implemented. The developer's role includes defining acceptance criteria, creating test cases, and ensuring the software behaves as expected. They work closely with development teams to identify and resolve software issues and improve product quality. Essentially, a Behat developer aids in refining the software development process for more reliable outcomes.