You should read this:

http://forums.khinsider.com/future-kingdom-hearts/110788-everything-we-know-about-unbirths-so-far.html

I have a (crappy) theory

Terra says he doesn't want to be "master" (whatever "master" is) and will only use his power (not sure if he means it in general or he actually has some sort of special power) for his friends to Xehanort. I'm assuming he'll be over to take over something but I'm not sure what obviously. I'm assuming he has some special power and it's some sort of power of darkness. In one of the screens he's talking to Malificient and laughs suspicously. I'm assuming he's being corrupted somehow. Probably by Xehanort. Anyway, in one of the screens Ven asks Terra to erase (ie. kill) him. Something tells me he's going to go corrupt by what might happen to Ven (Terra says Xehanort did something to Ven's heart) or he might just be consumed by the "Unbirths" or whatever (kinda how Riku was consumed by darkness, and in turn the Heartless...sorta) and just goes crazy. Probably explains why Ven nor Aqua weren't in KH1/KH2 (though Aqua might be in 358/2 Days). They probably got...killed. Also, you know the Dark Soldier wearing theh helmet and such in the secret ending of KH2: FM+? Something tells me that's Terra. I mean the body fits and like I said, Terra is probably consumed by darkness and tries to kill (he was attacking him, so...) Ven. Though what's weird though is the Dark Soldier was being fought by Terra and co. in the secret ending.

Though I'm probably wrong :P Speculation on KH games just...doesn't work.

Also, at SE's event on August 2-3 a new 358/2 Days trailer was shown and apparently the 14th member (Xion) has dark blue hair...like Aqua. I'm starting to think more and more Aqua is the 14th member and somehow managed to escape BBS's events somehow.