Varys

Master of whisperers to the Mad King and then to the man who overthrew him, Varys survived a change of dynasty by being indispensable to both. His little birds heard everything in King's Landing, and what reached the king's ear was carefully chosen.

It was Varys who brought Robert word of Daenerys's pregnancy across the sea, and thus Varys who set in motion the argument that ended Robert's friendship with Ned Stark. Whether he served the realm, a hidden claimant, or only his own long design, he never served Robert — he merely worked for him.