What is Ester's role in the play?

Aunt Ester, at the ripe old age of 287, is a source of much wisdom to the other characters in the play. Because she's lived so long and lived through so many important episodes in African American history, she has a unique perspective on things that gives her a certain moral authority over others.
People in the neighborhood regularly turn up at Aunt Ester's place looking for advice and spiritual sustenance. Aunt Ester acts as a spiritual healer or soul cleaner, providing a kind of moral absolution to those tortured by guilt and sin. Citizen Barlow is just one of many desperately in need of a good old-fashioned soul cleaning. He's been wracked by guilt ever since he committed a theft, and he needs Aunt Ester to help him deal with what he did and move on with his life.
She does so by taking him on a spiritual journey to the City of Bones, a strange, mystical place where he can achieve redemption. However, in order to get there, it is necessary to go back in time through history. Citizen must go back and admit his crime to the man from whom he stole. Only in this way will it be possible for him to cleanse his soul.
It is here that we see Aunt Ester's overall role in the play most prominently displayed. She shows us the importance of dealing with history as a means of understanding the present—and, by extension, changing one's future. She exists to remind us that no one need be a prisoner of their own history: if one confronts that history head on, no matter how troubled it may be, then it becomes possible to build a future life for oneself, free from the shackles imposed by a mindset steeped in the ignorance of the past.