Riltia was later confirmed to be a Willborn rather than the standard Illborn that we run into in place of monsters of a typical RPG setting.
I was just pointing out that I think if a sibling were to lose Soul Charge, but not fulfill the conditions for turning into Willborn that they would transform into a humanoid Illborn like Chief instead of a monster like an Undersucker.
So...because the one Sibling to turn into an Illborn turned into a specific subtype of Illborn, you think that Siblings who turn into a different type will look completely different.
What? Who care's about looks?
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.
What I'm saying is we know that Illborn can have more monstrous forms (ie Undersucker, Cool Crab) or normalish forms (ie Chief, Wilford (in case you question, this remember that Al's chat with Wilford began to imply that Wilford wasn't being attacked by Illborn like Al. Which further implied that Wilford lacked Soul Charge)). Willborn have one distinct more quality than being a simple subtype and that is that we cannot beat them in a normal fight, they need to be defeated in a Spirit Duel. So we can say that not all monstrous Illborn are Willborn because we can fight them just fine.
We know that there has to be some kind of reason for the different types of Illborn and that it might have something to do with Soul Charge. In the theory crafting answer, FFS does refer to a quote that seems to establish that Illborn might become less monstrous by gaining Soul Charge.
Now with Cain, FFS said that anytime we could fight him the concept of Willborn goes out the window. For instance, right now if we fight him... he can't go Willborn due to being unable to lose Soul Charge. The only known way to lose Soul Charge require access to the Soul Star.
This means that if we want to fight Willborn Cain one of two things needs to happen: a) He needs to get his Star back or b) We need to obtain his Star. Also add in that in option a, Cain would have to willingly drain himself of Soul Charge which he might never do even to spite us. So this alone could be the reason why. There's simply no way of obtaining Cain's Star and then draining it.
But it seems odd to me that we won't be able to obtain Cain's Star and we know that we can damage the Star to cause drain similar to what happened to Riltia.
So maybe if we did this Cain would just become a standard Illborn, because the condition to make him Willborn is fulfilled. 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? 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.