Who is the third...?

              "(...) Who is the third who walks always beside you?
                      When I count, there are only you and I together
                       But when I look ahead up the white road
                       There is always another one walking beside you
                       Gliding wrapt in a brown mantle, hooded
                        I do not know whether a man or a woman
                       -But who is that on the other side of you? (...)”