The Burger King/WMG

The King is The First.
After getting his ass kicked he really had no place to go.

The King wrote the song "What Is Love?".
He did so while he was on fire.

The King is cursed.
He doesn't seem to be alive in the regular sense, and he doesn't seem to enjoy his work. So, it's quite obvious that he's some guy who sold his soul to someone, who sold it to someone, who sold it to Burger King. Now that he's dead, they teleport him to some unusual place, and he's not allowed to leave until he advertises their food.
 * This also explains why there are multiple Kings, such as the Klingon King and the Cybertronian King; various people have sold their souls to him.

The King is King Arthur.
Like Arthur, the King possesses powers unlike mortal men. Not only that, but Arthurian legend states that King Arthur never died, but rather fell from power, and now waits for Britain's greatest hour of need. Until then, he just set up a fast food joint to make ends meet and to keep himself busy.

The only way the King can be creepier
Is if he was part weeping angel.

The King was once a Burger King employee
until poor working conditions caused his disfigurement thanks to poorly-contained, sizzling fryer grease. His vocal chords were removed so he couldn't speak out against the company, and he is now paid millions to be their mascot.
 * There is actually a certain amount of Fridge Horror involved in that theory. The King, when presenting people with food, is really trying to tell them how dangerous it is with visual aid, but because of his voiceless state, he is forever trapped with the dark knowledge of the truth about his corporate masters, never to be understood by the ignorant masses that continue to eat their poisonous food.

The King is Ronald McDonald from a dystopian future
In the future, as the world gets more P.C., fast food restaurants continually get blamed for the fattening of America. McDonald's, desperate to cleanse themselves of this image, completely revamp themselves, the board turns on Ronald McDonald in their anger, and ousts him from the company. But Ronald knows all the corporate secrets, so the restaurant's shareholders hire assassins to eliminate the clown, as well as the citizens of McDonaldland. Grimace, the Hamburglar, Mayor Mc Cheese, and most of the rest are killed in the ensuing holocaust. Although Ronald (and close ally Birdie) survives the assassination attempt, the experience leaves him severely scarred, both physically and mentally. He decides to create a new restaurant, one where he can be in charge again. He travels into the past and adopt a crude disguise of a plastic king mask, and begins to form his own franchise to counter his former restaurant, thus fulfilling the ancient Sumerian prophecy (circa 1250 BC) that "When a clown falls, a king shall rise." Birdie gets extensive plastic surgery, dyes her pigtails red and renames herself "Wendy", but that's another story...
 * Impossible. The Sumerian people were absorbed by their neighbors by the 1800s. How could they possibly write a prophecy in 1250, hm?