A Knockout developer is responsible for creating dynamic and interactive web applications using Knockout.js, a JavaScript library. They use its Model-View-ViewModel (MVVM) pattern to separate the application's data model, user interface, and actions. They bind HTML elements to model data using declarative bindings, making the UI automatically update whenever the data model changes. Knockout developers also handle dependency tracking and declarative bindings to maintain complex UIs and create rich, responsive display and editor user interfaces. They may also work with other technologies like HTML, CSS, and other JavaScript libraries to build comprehensive web solutions.