Bojack is a currently unseen pirate. He was the main villain in the movie Bojack Unbound. He was sealed inside King Kai's planet (which King Kai claims is the reason for the intense gravity on his planet). Being a "space pirate", he speaks with a stereotypical pirate accent, starting all his sentences with "Yarrrrgh".
King Kai is not appreciative of Bojack at all, constantly demanding the pirate's silence even when he, Bubbles and Gregory try to console a lonely Kai. Bojack is seeking revenge on all Kais and will destroy anything to until he is satisfied.
In the original it is never revealed why King Kai's planet has high gravity, he just says it has high gravity and that the planet is small.
He was originally voiced by Vegeta3986, but since he left, his current VA is Antfish.
While TeamFourStar may have intended citing Bojack as the reason behind the increased gravity on King Kai's planet as a joke, the fact that destroying a planet would require overcoming a planet's gravitational binding energy makes this a somewhat plausible explanation, as far as anime logic goes.