One of each for the moment, anyway, but more is likely to come to mind...

Question: has anyone unlocked any characters other than the Major and the Hunter, and if so, how?

Other thing: I want to know if anyone else has found the Magic Skull as useful as I have. I'll explain later if not, but suffice it to say, I always hang onto it now once I find it.

I've unlocked all characters, you just have to be lucky enough to find a prison-cell room (the ones with the books). I don't think there's anything special to do to find them, it seems to really be just luck.

I did pick a magic skull once but I don't remember what it does, my favourite item is the heart and I usually stick with it when I find one.

Heart/Skull is the combo I stick with once I have it, yeah.

Best I can tell, the random world is the only one I don't have the story fragment for, so I've just started playing through that repeatedly to try and (I assume) get the last guy, whoever he is.

The last character is a dude dressed as a dragon, and he has the bubble gun (lv2 I believe).

Finally finished the game yesterday, I dunno why I was expecting a bad ending, I'm kinda disappointed actually.

I do have a question...
Do you have figured out what the various symbols do?
So far, I've found the what the following do.

Heart - Gives you two hearts
Satanic - Poisons enemies around you
Eye - Temporarily freezes enemies
Time - Extends time for the level (?)

I really enjoy all those Bubble Bobble references in the game, like the parachute, the bubble gun and.. well... the green dragon with the bubble gun. :D