What? Who care's about looks?
When the difference in looks is the difference between more or less makeup? Basically no one.
When the difference in looks is the difference between a human and a horrible eldritch monstrosity? Basically everyone.
I'm thinking along the lines of Nobodies from Kingdom Hearts. A Person who loses their heart may create a Nobody if their Heart was strong enough. If their Heart was even stronger yet their Nobody will have their Humanoid form rather than the standard run-of-the-mill monster form.
Of course...
1. Heart =/= Soul Star
2. This isn't Kingdom Hearts
True, it is not. But since the Person/Sour Star/Illborn thing parallels Kingdom Hearts's Person/Heart/Heartless/Nobody thing, it made good example to illustrate with. I am not saying that it works exactly the same but I was using it to explain what I was thinking better since you seemed to misunderstand what I was saying.
3. Riltia lost her Soul Star,
We don't know that Riltia lost her Soul Star. She lost her Soul Charge which caused her to transform.
and she turned into a horrible Illborn monster. Why wouldn't other Siblings do the same?
Because we were told by FFS that Willborn are specifically different from the standard Illborn. We have no reason to believe the siblings will become Willborn, because we don't know how a Willborn differs from a standard Illborn. There have been several theories, but none confirmed. So its is reasonable to think that siblings are as capable a turning into a non-Willborn Illborn.
You can handwave all you like, but unless you come up with a concrete reason why Riltia's case was so horribly, horribly away from what could happen elsewhere, you're just blathering.
We haven't been able to get that much information yet as the only two Willborn we have seen are the only two transformations we've seen. But we have seen many different standard Illborn both monstrous and (for lack of a better word) normal.
So maybe if we did this Cain would just become a standard Illborn, because the condition to make him Willborn is unfulfilled. Now in that case, if we did that would we come back to Cain's boss room and find a Hellyfish floating in his place?
No. Why the hell are you strawmanning me? That's like if I asked if you would expect to see Wilford in Cain's throne room.
Are you saying that there is some reason that Cain wouldn't become a Hellyfish type Illborn? I am legitimately asking a question here, not Strawmanning. Like I said my theory is he wouldn't, but like you said I have no proof hence it's a theory. We have no idea if Cain satisfies any condition (to become Willborn or to become a nonmonstrous Illborn like Chief) so its all speculation.
I just think that siblings probably already fulfill whatever condition that causes Chief to look more-or-less normal and we'd find Cain looking more or less the same. Naturally, I've got no proof at this point but I find it a reasonable enough idea for the present.
And I find it a reasonable idea that the one example we have of an Illborn sibling, Willborn or no, is not horribly un-representative of the norm, and hence horribly monstrous Sibling Illborn are not something we can discount because of Kingdom Hearts.
I never said to discount horribly monstrous Sibling Illborn because of Kingdom Hearts. I'm also thinking that Willborn will always look monstrous... judging from the fact that each character has a specific Willborn form and that Willborn are supposed to be a bad thing.
I just said that if a Sibling were to become a standard Illborn that I think they would possibly fulfill whatever condition keeps Chief in a non-monstrous form rather than turning into the monstrous Illborn we seen in the dungeon levels. I do know this doesn't have to be correct, so I'm not arguing this point simply explaining it. It seems from your replies that you misunderstood what I said.