A Hystrix developer specializes in using the Hystrix library, a fault-tolerance library developed by Netflix. Their main responsibility is to ensure the resilience and fault tolerance of distributed systems. They handle latency and fault tolerance logic, ensuring that remote systems and services can operate and interact reliably. They design, code, and implement 'circuit breakers' to prevent system failure during network issues or high traffic. They also monitor, isolate, and manage dependencies on distributed systems to avoid cascading failures, ensuring that applications can continue to function even when some components fail. They contribute to the robustness and resilience of the system.