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 regards Jillian as an absolutely vital ally. In fact, he suspects that she may be the only person who can save Erfworld from total destruction. He also likes her personally, regarding her as his "sweet, cranky, warrior-Queen". He first met Jillian at the moment when he realized that Stanley posed a far, far greater threat than anyone had previously believed. Because of his ability to secretly observe events through his bats, Don may have seen far more of the Battle of Gobwin Knob than anyone realizes. He understands that Stanley poses both an ideological threat (because of his claim to a new Titanic mandate), and a strategic threat, (because Gobwin Knob has weapons that can wipe out an entire armies). This new awareness has forced him to reassess many things, including his views on the importance of Royalty.
Don may have been watching Jillian though his bats since long before he met her. It's not clear exactly how much he knows about her, but he certainly knows that she is a young Royal with extraordinary combat abilities who has good reason to hate Stanley and who has demonstrated some capability to predict Stanley's actions. After meeting her in person, 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 was 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. It was after this point that he dramatically increased the resources he was sending to Jillian in order to expand Faq's military.
Don King is used to being disappointed by his Heirs. He disliked his Royal daughter, and his Royal son tried to betray him. So after both of his children croaked, he appointed a minor Noble named Caesar as his Chief Warlord and [Heir]]. At first, Caesar seemed much better qualified to rule than either of Don's children had been. He was a capable Chief Warlord and earned the respect of Transylyito's Warlords and troops.
Unfortunately, while Caesar was capable of dealing with normal problems, he vastly underestimates, in Don's view, the magnitude of problem posed by Gobwin Knob. Don is probably also aware that Caesar feels a bitter antagonism towards Queen Jillian of Faq, despite the fact that Don has made it clear that Jillian plays an important role in his plans. These major policy differences have apparently persuaded Don that Caesar is not qualified to rule, because he has ordered the creation of a new Royal Heir. Don has not yet indicated that he intends to replace Caesar as Chief Warlord, though Caesar suspects that Don tried to croak him. As he watches to see how Caesar reacts to the creation of a new Royal Heir, Don will likely remember how his son tried to replace him. The possibility that Caesar will try to betray him is not likely to catch Don by surprise.
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.
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".