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.
Don King first met Jillian at the moment when he is reassessing his views on the importance of Royalty. He realized that Stanley posed both an ideological threat (because of his claim to a new Titanic mandate) and a strategic threat (because Gobwin Knob had just used an unknown weapon to wipe out an entire army). Jillian probably interested him because she is a young Royal with a great deal of combat experience who had good reason to hate Stanley and who had demonstrated some capability to predict Stanley's actions. He decided to help her rebuild Faq. But then, he learned at some point that Ansom, whom he knew Jillian loved, was not, as previously thought, croaked. Instead, Ansom is leading Stanleyès forces as his Chief Warlord. This may have lead Don to question how Jillian would react to Ansomès apparent decision to support Stanley. Could Jillian be trusted?
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".