A libev developer is responsible for implementing and managing software applications using libev, an open-source, high-performance event model library. This includes writing clean, testable code, debugging applications, and integrating libev functionalities into different software. They also optimize applications for maximum speed and scalability, while ensuring its responsiveness and efficiency. Additionally, they collaborate with other team members to define, design, and ship new features. Their role involves understanding the intricacies of libev, including its multiple backends, timers, signals, and other mechanisms, to provide efficient, scalable, and fast event-driven programming solutions.