A Job Control Language (JCL) developer is responsible for creating, testing, and debugging JCL scripts in mainframe environments. They work with JCL to manage and control the execution of tasks, or 'jobs', such as calling programs, allocating resources, setting up inputs and outputs, and handling errors. Additionally, they may be tasked with optimizing job flows to improve efficiency and reduce system usage. The developer also often collaborates with other IT professionals, such as system administrators or programmers, to ensure smooth operations and troubleshoot any issues or errors that may arise in the mainframe environment.