Who is Brother Constantine from Brother Odd and what is their importance?

Brother Constantine is the only spirit monk to inhabit the Abbey, Brother Constantine manifests himself occasionally by ringing the bells in one of the towers in frustration, or making rare appearances elsewhere in the Abbey. Unable to leave while the circumstances of his death are still so misunderstood, he remains, trying to give Odd clues as to the truth. What none of his fellow Brothers understands is that Brother Constantine did not commit suicide.