All right, then think about this: In a world where being a Sourcerer is feared and hated and basically a death warrant on your head, what reason would the Sourcerers have for recruiting a powerful fighting companion of unknown allegiences?'
If they do not trust this companion enough to say that they are themselves a Sourcerer, why would they trust them enough to travel with them? Such a companion could become a threat or an enemy if the truth were to be discovered.