Gracksa talks about the science of the Kirby universe: A post
Soul forms in the Kirby universe occur when a being of (inherent) considerable magical power is possessed or taken of an even greater power after death. In most cases, it is not wilful: Drawcia was possessed by 02, Magolor was possessed by the Master Crown, and only Marx's transformation seemed to be even remotely conscious. He absorbed some of NOVA's parts (somehow - I've got no idea how he did it while dead, but hey, stranger things have happened in this universe).
To list off how each form occurred:-
- Drawcia (whose inherent power already came from a deal with 02 - she was originally just a painting, nothing more) was possessed after death by 02, who at that point had decided he wanted to interfere with the situation personally.
- Marx (a great majority of his power came from his wish to NOVA) absorbed some of NOVA's parts after his death at Kirby's hands, some unknown period of time afterwards. We all know how powerful NOVA is.
- Magolor (who has little inherent power himself, hence seeking the crown) was possessed forcefully after death by the Master Crown.
Note that all of those transformations happened after the character died. It is impossible for a character to enter their soul form while still alive. Being really angry or really distressed like any generic anime super-form is not a plausible reason (I've seen quite a few people do this with Marx Soul and it kinda irks me).
For all three of those characters, it's very obvious that they are dead in their soul forms. Drawcia blew up, Marx was seen floating dead in space, and Magolor was having his very corpse puppetted.
By extension, it is also impossible for the characters to enter that form willingly. They would have to first die and then be possessed of a greater power. The only character of the three who can be guaranteed to enter their soul form after death (and this is massive headcanon) is Marx. The way I see it, because a vast majority of his power came from NOVA (and he absorbed NOVA's parts after death), and NOVA is still alive, they have this kind of magical connection which means that NOVA would not have to instil his power in Marx to bring on the change. Does that make sense? For Drawcia or Magolor to enter their soul form, they'd have to have some great power take them over after death.
It's also heavily implied that a soul form is not a pleasant thing. Drawcia is constantly screaming throughout that phase, and Magolor is dead. His body is simply being tugged about by the crown and the most he can do is watch. Marx seems to be the only one who looks like he's enjoying it, and that could very well be because he's batshit insane like that.
It's pretty obvious that characters do not have control of their actions while in a soul form. Drawcia attacks in a much more rapid, aggressive manner than her usual pattern (in fact, all of them do, take note), and Magolor's actions are being dictated entirely by the crown. Again, Marx does not seem to have anything apparent controlling him, but similarly, by this point he is so derailed that his actions are entirely different to what he would normally do. Not in control of himself.
When in a soul form, the character undergoes some physical changes - sometimes drastic, sometimes not - according to the power that possessed them. Drawcia became spherical and took on the single, huge eye that is 02's signature and Magolor's body changed dramatically to what was basically his core energy being held by the crown. Marx's transformation was not so huge, and that seems to be because he absorbed NOVA's parts willingly; perhaps with the other two, it was because they were possessed forcefully, and so could not control what they looked like? Either way, Marx obtained an amulet made of the parts he absorbed, among other smaller changes.
So there, that was my rant on the science of soul forms. Aside from the parts noted as headcanon, every bit of that is canon. However, if you spot a bit and think I'm wrong, feel free to point it out! And if you read that wall o' text, thank you.))
