As the theory goes, King Boo, who first appears in the video game Luigi’s Mansion, is the ghost of Bowser, who died in Super Mario 64.
In the Nintendo 64’s Super Mario 64, the last (non-spinoff) Mario adventure game to be released before the Gamecube’s Luigi’s Mansion, if you achieve the 120 star ending, Mario appears to defeat Bowser in spectacular fashion.
Roughly halfway through Luigi’s Mansion, a ghost fortune teller says:
“Wh-what is this!?
How can this be?! I see the hideous form of Bowser!
Is Bowser somewhere in this mansion? I cannot believe it…
And yet I see it…
I thought that Mario had soundly defeated Bowser!
Has King Boo somehow revived Bowser? This could be horrible! Well, for you.”
What’s so hideous about his form? How hard did Mario defeat Bowser? Did Bowser die?
In Luigi’s Mansion, King Boo is the antagonist and final boss of the game, and he fights Luigi in a mechanised Bowser suit. Just before the fight, however, Luigi confronts King Boo who mentions delighted he is to have trapped Mario inside a painting because of “all the trouble he’s caused” King Boo. The problem is, King Boo had never appeared in a Mario game before Luigi’s Mansion. We must assume King Boo is referring to all the trouble that Mario caused for Boo when he was alive, when he was Bowser. From this we can conclude that King Boo is the ghost of Bowser, and he constructed a mechanical version of himself to fight Luigi.
Moreover, the Dry Bones characters are known to be dead Koopa Troopas possessed by their own spirit in the form of Boos. Thus, it should follow that if Bowser, the king of the Koopa Troopas, dies and is resurrected as Dry Bowser, he would be possessed by his own spirit in the form of King Boo.
This theory does suffer from some holes though, mainly because it is difficult to establish which Mario games are canon. For instance, both Bowser and King Boo appear as playable characters in Mario Kart Double Dash, but Mario Kart games are regarded as spinoffs and not canon. Another problem is that King Boo appeared in a canon game whilst Bowser was alive in the same game. This occurred in a casino level in Super Mario Sunshine. However, it is argued that Mario summoned King Boo from a portal or alternate universe, meaning his timeline did not cross with Bowser.