A HelloSign developer primarily works with the HelloSign API to integrate electronic signature functionality into websites or applications. They design, code, test, and debug to ensure seamless user experience in document signing and management. The developer might also customize the eSignature workflow, enabling users to sign documents digitally, track document status, and store signed documents securely. Additionally, they may handle API-related issues, ensure data security, and maintain the integrity of the API. Their role also includes keeping up-to-date with new features and updates from HelloSign API, and implementing them to improve application performance.