Unversed
The Unbirths are the enemies Terra, Ven and Aqua will face in the upcoming Kingdom Hearts: Birth by Sleep. Strangely enough, the Unbirth's emblem resembles the emblem of the Heartless. This could mean the two have some kind of connection.
Timeline
Although little information is available, these creatures have been thought to be extinct, for at least the time since the events of Kingdom Hearts (game), which occurs 10 years after the appearance of the Unbirths. They have also been linked to both Heartless and Nobodies, as neither apparently existed during the time of the Unbirths.
Meaning
First Theory
The name Unbirth resembles the words "Nobody" and "Heartless" in a certain manner, like so:
Nobody = Bodiless
Heartless = Without hearts
Unbirth = Not born
Their way of production could be connected to Nobodies and Heartless. While the Heartless are shown as dark beings and the Nobodies are twilight beings, it could be said that the Unbirths are beings of light, simply evil. The Unbirths could have a connection with the Heartless, and the Nobodies as they all seem to lack something.
What the three seem to lack is a part that makes up a complete person (Heart, Soul, and Body). The Nobodies lack a body, and Heartless lack a heart. This could mean the Unbirths lack a soul which might explain why they're enemies as they might have no free will of their own. This theory has yet to be confirmed as it is speculated that a soulless body is a dead being.
Second Theory
Another theory is that they are each corruption of parts of a complete person.
Heartless = Hearts = animal-like, instinctual
Nobodies = Body = "adults," calculating
Unbirths = Souls = children, impulsive and driven by feelings
Heartless are, rather than having no heart, are corrupted hearts consumed by darkness. Heartless would generally be animal-like, living off instincts. Nobodies are, rather than bodiless entities, are living bodies seeking to restore their hearts and find a place among light or darkness. Nobodies would be systematical beings, of an "adult" thinking. Based off this, one could say that Unbirths are souls of the realm of light who were not born into corporeal bodies, and would have a childish impulsiveness.
Third Theory
Another theory states that they are beings of a realm of Chaos, as Heartless are beings of a realm of Darkness, and Nobodies are beings of a realm of Nothingness. If so, this could explain why they're dangerous; they are unpredictable beasts with a hostile attitude, and simply destroy things because they can.
Fourth Theory
The fourth theory is that Unbirths are a kind of mix between a Heartless and a Nobody, since they appear similar to both. This is plausible since, as previously stated, Heartless and Nobodies did not exist during the time of BBS. It could be that, for some unknown reason, the Unbirths would split into Heartless and Nobodies. As all the other theories, this would be proven or disproven through BBS' story.
Bosses
Although few scans have appeared and no specific enemy names are known, there is one case that is quite the opposite. A boss known as the Wheel Master is known to exist.
Appearance
2 types of Unbirths have been shown in scans. A small, skinny creature, blue in color, and a large creature also colored blue. Both feature odd red eyes and large, zig-zagging antennae/ears. In a new scan, both flower like Unbirths and pot-like Unbirths are shown. The flower looks like a potted flower with vines for arms that seems to spin around like a top, while the Pot is simply a red pot with bat wings.
Allegiance
Some think their alliegance is to just evil itself having no cause. Others think it may be used by a boss of the new game. that means we will have to wait and see if they are pure evil as suggested,saying they have no soul, or whether they are controled by an unknown entity.
