One person is responsible for managing and controlling the Product Backlog. That person is referred to as the Product Owner. ... The Product Owner is one person not a committee. ... The practice Scrum adds is that only one person is responsible for maintaining and sustaining the content and the priority of a Single Product Backlog.
—Ken Schwaber and Mike Beedle1
Customer: A role on the team for choosing what stories the system has to satisfy, what stories are needed first and what can be deferred, and for defining tests to verify the correct functioning of the stories.