Rivkah joined the Prophet because her husband, a Rus nobleman, was a believer. When he died on the journey north, she stayed with the Prophet, pondering what her place was amongst the believers.
When she saw the valley, she knew she had found her purpose. She dreamed what Kitezh could be. She laid those dreams out to be built. It is because of Rivkah that Kitezh became not just beautiful, but livable.