King Boo...Who are you?

There's no way that's possible, since Bowser was shown to be alive in just about all the games that King Boo has been in. Plus, Bowser's and King Boo's personalities are completely different, so there's no way they're the same person.
 
Daggett said:
King Boo is the ghost of Morton Koopa Sr. :P

AKA Bowser's dad
 
I think Boos aren't the spirits of the dead. Dry Bones are reincarnated Koopas. Dry Bowser is more likely to be Bowser's dad.
 
Oaktown Mojo said:
I think Boos aren't the spirits of the dead.
Oh, but they are. This is supported by what Lady Bow said in Paper Mario: "I swear on my own grave! Mario's innocent!"
 
If we throw in Boss Boo into the mix and the fact that they have separate identities at times but the same ghost at other times confuses things.
 
My guess is that they're spirits of humans. Well, at least according to what King Boo said in Super Mario 64 DS. He said something along the lines of, "You'll die of fright before THAT ever happens! And when you do, you'll be one of us!"