A Knockout.js developer designs and implements user-facing features for websites and applications using Knockout.js, a JavaScript library that helps developers create rich, responsive display and editor user interfaces with a clean underlying data model. They utilize its declarative bindings and automatic UI refresh system to create complex interactive features. They are responsible for building reusable code, ensuring UI/UX designs are technically feasible, and optimizing applications for maximum speed and scalability. They also troubleshoot and fix bugs, maintain code quality, and collaborate with other team members to define, design, and ship new features.