Whisper Dan is character who first appears in "The Silent King", ultimately becoming the character referred to in the episode's title. He appears to be a silent servant to the king and queen, as some kind of automaton or golem. In Xergiok's castle his job is to turn cranks. He does not appear to have his own will or personality.
Gummy orders him to turn the crank that cuts the king's bed in half and turns it into a bunk bed. Later, Finn gets him to turn the crank to open a sensory deprivation chamber that Jake is lying in. In order to escape the obligations of royalty, Finn disguises Whisper Dan as himself, by putting a cardboard box with a perfectly-drawn face on his head and the crown on top. This apparently fools the goblins, who continue to serve him as their silent king. The goblins do not seem to notice it is Whisper Dan even though it is clear he is not Finn.