Varys, sometimes called "the Spider", is the Master of Whisperers (spymaster) of King Robert's Small Council. His 'little birds' are planted all over Westeros, as well as further afield.
A fat, bald eunuch with a fondness for fancy clothing, Lord Varys is of simple birth (at least according to himself) but has risen to become the head of a large web of spies and informers. The mere nature of his work makes him seem untrustworthy to many, and this is compounded by his habit of disappearing and reappearing out of nowhere, and giving his help to seemingly anyone who asks, making his true allegiances hard to discern. He is nothing if not enigmatic.