A Cap'n Proto developer designs, builds, and maintains efficient and reliable systems using Cap'n Proto - a data interchange format and capability-based RPC system. They utilize its serialization and procedure call features to implement data structures and interfaces, ensuring fast, efficient, and secure data exchange between different parts of a system. They also optimize memory usage, reduce data copying, and enforce access control. Additionally, they handle the tool's backward and forward compatibility to manage system evolution. Their role may also involve resolving issues, debugging, and enhancing system performance.