Indeed, I think to begin at the end will reveal the beginning and purpose of the Unbirths.
They don't exist at all in the KH universe after BbS so wth happened?
[I still think they're a very early and feral form of Heartless, but that's just me...]
I think it's more of an evolve sort of thing over time, We have noticed already that there was a Kingdom Hearts in BBS. It's like there are higher forms and lower forms of them just like nobodies except similar to what the heartless are. It kind of reminds me of the DS except I think there's a different story as to why the DS is the way they are. The unbirths would be not birth so you would think they wouldn't be born but Heartless have Hearts and Nobodies have bodies so an unbirth could have a birth just less of one so it wouldn't be a natural one and they were just created as brought up before.
The Unbirth are wild.. So I think they are more like heartless than nobodies.. Nobodies can think but heartless is only instinct..
I think they are more like Heartless than nobodies because nobodies are just the body and the soul when a heartless would be darkness, The heart, and their natural instincts. 2 features of a nobody and 3 features of a heartless. But the 2 to 3 thing is only if you look at how they are in my way.
Namine is a nobody except she just happened therefore she isn't what you would call naturally born but less of a birth she was just created,right? As spoke of before. And obviously there was a process to go through first so no giving me bullcrap again. So to be created as some type of form, take nobodies or heartless for example they weren't born it was just more like a transformation of a person.
The new enemy isn't a Heartless or a Nobody and Nobodies and Heartless weren't around until after BBS about a year after BBS when the young Xehanort had created the first one. So unless there were different enemies or creatures before BBS the process wasn't by some sort of evolving type of state. They could be like nobodies, Nobodies take lower and Higher forms, They might be like the DS and keep a heart full of darkness. The DS would be a higher form. When the heart is taken by darkness the body seperates as body and soul and the Heartless just keeps the heart and goes by it's instincts so a heartless with a body and soul that can take higher and lower forms. Humanoid dark form to something like dusks combined with heartless forms. It wouldn't really be a Heartless but it would be filled with darkness so would a person not transform if they had a weaker heart in such a process? I think they would but if they are unbirths then they must have been created in such a process(transformation) unbirth=Less of a birth not never born at all.
When Nomura says the birth of Roxas it doesn't make all that much sense unless he's more specific because Roxas is Sora and it's just him without his heart so it would be like saying the birth of Sora but Sora was able to take a humanoid form when Kairi revived him but how does that make him a walking heart when she revived him. He had gone by instinct at all when a Heartless and could use his mind and heart and then later Roxas returns to him as his soul instead of as his original body and soul(The other nobodies only had heartless and themselves they didn't have their heartless revived like Sora so that only his soul would return and he would be normal again just like that. They had to try and get hearts not just have their souls return as simply as Roxas and Namine did.)So it's as if they are seperate beings but the same person, The fact that they are the same wouldn't really effect Sora once he returned either so I think that's what he's trying to say when he brings up birth. With Namine kairi didn't even need to seperate her heart and body, her heart just returned to her,right? So that just made Namine happen when sora released her heart and became a heartless his body and soul left as Roxas and so Namine was made from Kairi's Heart and Soras body and Soul as well(I think that would explain Namine's hair color.) which made what i'm guessing is another of Kairi as they are the same when they return there isn't really a difference. They could see eachother face to face as that is the obvious and you don't have to reply at the seeing eachother face to face thing by saying you already knew that far... Cause you likely do.
I'm not saying nobodies are unbirths though, They are different because Unbirths may actually keep hearts and come to be in a different way. The unbirths might actually have minds as well but I don't think they can control themselves as you guys brought up, They are wild. But at higher levels they'll probably be able to maintain better control.
So those are my thoughts and theory on the unbirths...