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's also persuaded Jillian to pop a Royal Heir for Faq, in return for certain concessions, and begun creating a new Royal Heir for Transylvito.
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.
After discovering that Charlie had attempted to manipulate Transylvito using Jillian as his agent, Don has been forced to reassess his perspective once again. Between the serious threat Charlescomm poses to his side, Parson and Jack's slander, and Jillian's own unreliable nature, Don King now fears that Jillian wasn't who he thought she was after all.
Chief Warlord Caesar Borgata
Don King is used to being disappointed by his Heirs. He did not get along with his Royal daughter, and his Royal son tried to have him assassinated. So after both of his children were croaked, he decided to change how his kingdom was organized and simplified things. He dropped some of the fru-fru of nobility and kept what was useful. He appointed a minor Noble named Caesar Borgata as his Chief Warlord and Heir, Don King said he did so because Caesar earned it. At first, Caesar seemed much better qualified to rule than either of Don's children had been. He was a very capable Chief Warlord and earned the respect of Transylvito's Warlords and troops on his own merits and accomplishments.
However, following the Battle for Gobwin Knob, Don King has again started to rethink the importance of royalty. Stanley claims a titanic mandate and that royalty is obsolete. Especially after the croaking of Queen Bea of Unaroyal, he places much greater importance of royalty. Unfortunately now he has a low ranking noble as an heir. The close relationship the two enjoyed before the Battle for Gobwin Knob has been divided by disagreement. Caesar is opposed to Don King bleeding all of Transylvito's resources to Faq. Transylvito is threatened on all sides because they can no longer defend themselves against kingdoms like Carpool who previously posed no threat.
In Don's view, Caesar doesn't appreciate the magnitude of the 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 Jillian plays an important role in his plans. Don has rethought the wisdom of downplaying the importance of royalty and appointing Caesar heir designate. He has decided to pop a new Royal Heir to take Caesar's place. Don has not yet communicated with Caesar whether he will also be replaced as Chief Warlord. Caesar seriously suspects that Don tried to have him croaked by sending him with a small force on a suicidal attack against a heavily defended Chocula. Don himself openly hopes Caesar will be croaked in the near future on his increasingly dangerous missions.
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".