Two main goals of Agile Sprint are building new functionality and building new knowledge. Building new functionality seems obvious for sprints, but teams also need to focus on building knowledge.  Each sprint should end smartly from its inception.  A UI designer should focus on both of these goals. Within a sprint, a UI designer helps in coding and testing of his/her design while simultaneously, he/she focuses on the next design feature to be built. A UI designer's top priority should be working in the current sprint. If a developer or tester needs any clarification regarding the design than he/ she should immediately halt the work and attend to their queries. But then one might say that every member of the team does so. Be its analysts, architects, database designers. The difference lies in the amount. Designers spend majority of time in building knowledge. To know more, read the article by Mike Cohn (founder of Mountain Goat Software), at: https://www.mountaingoatsoftware.com/blog/incorporating-ui-design-in-scrum-sprints