“Who is Gloom? He is everybody, and nobody. Perhaps you, perhaps me, perhaps the man sweeping the foyer. Nobody knows, he came seemingly from nowhere but I can tell you I fear him. And I’m the one set to catch him.”
An Agent of the Crown

A (former?) member of house Channicut, apparently Gloom entered Amber during the Patternfall war, by reports he is a talented shape shifter able to seamless switch personalities. He was thought to be killed, but his body reverted to an androgynous puppet. Since then there is new evidence of his activities.