A Mocha/Chai developer is responsible for creating and implementing automated testing processes for web applications. They use Mocha, a JavaScript test framework that runs on Node.js, to set up the testing structure. Chai, a BDD/TDD assertion library for Node.js, is used for writing test cases that verify the functionality and performance of the code. The developer works closely with the software development team to understand the requirements, identify potential issues, and ensure that the software product is free from defects. They also maintain and update test scripts as per changes in the application.