Way of the Pilgrim by Gordon R Dickson

Shane, a gifted linguist, has spent his life learning the language of the old and powerful alien race that has conquered Earth. He has learned it so well that the interstellar masters, old hands at enslaving planets, regard him as a valuable servant.

But Shane has a secret. One day, in a rebellious moment, he invented The Pilgrim: a mysterious figure who incites rebellion and vanishes unseen, leaving a distinctive icon behind him.

Now the human underground is preparing to rebel. Shane knows how hopeless their rebellion will be. He knows, as well, that he will be unable to keep himself from taking part.