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.
It isn't clear if Don King can fly, though most Transylvito Vampires are able to.
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 Stanley's forces as his Chief Warlord. This may have led Don to question how Jillian would react to Ansom's apparent decision to support Stanley. Could Jillian be trusted, or would she join forces with her lover Ansom? It may have been concern about which side Jillian would fight on that caused him to demand that Jillian pop a Royal Heir, because producing a Heir would dramatically slow down Faq's ability to pop other units. When Don learned the truth about Ansom from Queen Bea's final message, he probably realized that his assumptions about how Jillian would react to Ansom were wrong. If he believes she will regard the new Ansom as an abomination, then Don is likely to trust Jillian more than he did before he learned the truth about Ansom.
Count Vinny Doombats
Vinny did not visit the city of Transylvito until after his 200th turn, suggesting that Don did not originally regard him as vitally important. At some point, Don apparently realized that Vinny had some talent for diplomacy. (The two assignments we know Don has given to Vinny were to work with the Royal Crown Coalition, and to act as liaison with Queen Jillian. Both positions require diplomatic as well as military skills.) Don had a high enough opinion of Vinny to listen to his claim that Jillian knew where Stanley was going, but he also gave Chief Warlord Caesar orders which he did not share with Vinny.
Don's recent reappraisal of the importance of Royalty has probably raised Vinny's status somewhat. He is a Count, which means that, in terms of his position in the nobility, he outranks Viscount Caesar.
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".