Don King is the Ruler of Transylvito. He is a royal. He persuaded Jillian to become Queen of Faq, and provided her with the money needed to rebuild that side. He is currently negotiating with Jillian the terms under which Faq will pop a Royal Heir.
He seems to trust Vinny a great deal.
It isn't clear if Don King can fly, though most Transylvito Vampires are able to.
Real World References
He shares his name with Don King, a famous boxing promoter.
"Don" is a title ostensibly held by the rulers of mafia families (but certainly used in many movies and television programs), reinforcing the mafia imagery associated with Transylvito.
His appearance closely resembles Marlon Brando in his overweight later years. Brando portrayed Vito Corleone in The Godfather, a famous crime film. This is reinfored by the description of Don King's voice being "airy".