Terra is pretty much confirmed to be a darkness. When Mickey is counting up the people for the light he says "Sora, Riku, and I...My three friends...and..." when he says his three friends, he's refering to TAV. But then later MX says, "Yes, very perceptive little king. But Sora and another one of yours belong to me, so you are still three guardians short." Now of course one of the three MX spoke of is the simply the last spot in the 7 that has yet to be filled by anyone. But when he says "Another one of yours" he's refering to Mickey's list and the only person that would make logical sense is Terra.
Obviously Terra isn't possessed by MX directly now. MX being an old man is evident of that. What I believe has happened is that we are seeing the opposite of how Roxas and Namine came into being. When Sora released his heart and Kairi's heart, his body was forced to split into two nobodies. However in this case, I believe when Xemnas was destroyed, his body was forced to split into two because he had to return to both Terra's heart and MX's heart, thus restoring them both. However MX of course left a fragment of his heart inside of Terra and considering that MX was in Terra for so long, we can assume this fragment is larger and stronger than the ones in Saix and Xigbar, as such Terra is still under MX's control, just not possessed by MX's primary heart.
So in conclusion I think we can safely add Terra to the list of darknesses.
Repliku has been hinted to be one due to that scene in Monstro where he is wearing a organization coat. Riku assumes when he saw Ansem and now Repliku it was his darkness inside making them appear as illusion. But obviously he finds out that is not so in the case of Ansem. So it should be the same with Repliku.
Vanitas appears next to YMX in La Cite Des Cloches. I think Vanitas is among the 12 hooded figures, but not there physically yet. I think YMX probably stopped by in BBS to take in Vanitas' damaged heart just as Sora took in Ventus' damaged heart. So I think when we see Vanitas next to YMX, it's a projection of him coming from YMX. As such it may be that one of the hooded figures setting next to YMX is another projection of Vanitas coming from YMX.
Finally we have Dilan and Even. They were the only two out of the ones restored at Radiant Gardens that were unstable. Why were they unstable? I think they are members of the darknesses. When Lea leaves Radiant Gardens to meet up with Mickey, there is plenty of time between then and the gathering of the darknesses at the end of the game for Dilan and Even to have escaped and joined up with them. Plus we never see Radiant Gardens again in the game after Lea left.
Xaldin in 358/2 Days even said he didn't want a heart, he just needed one because there are certain things only beings with hearts can do. He would mock other members for acting like they were showing emotions, and he was obviously well versed in manipulating a heart as seen in the case of Beast. Not to mention you can clearly see his ears starting to turn into the elfish MX years.
With Vexen we have the fact he was in charge of the replica program and obviously that takes a level of trust to give him such an important job over the Organization's research. Wouldn't be too much of stretch to believe Xemnas made him that because he's one of the people meant to be a darkness. Or perhaps Vexen is going along with it because he's afraid of Xehanort. He clearly was afraid of Xemnas. He may submit to MX's will because he believes MX is going to win in the end and he wants to be on the winning side to save his own skin.
I think in the end we'll see Lea, Ienzo, Aleaus, Demyx's somebody, and Luxord's somebody being the organization members that will side with Sora. Xigbar, Xaldin, Vexen, and Saix being the ones that side with Xehanort. (Although I could see Saix betray him later). And I think Marluxia and Larxene's somebodies will not want to side with either, but will help Sora for the time being since they share a common enemy in Xehanort.
As for the lights, I have a question of my own. Can Kairi really be one of the Guardians of Light? She is a Princess of Heart, but the Guadians are chosen the the lights of the X-Blade to protect the Princesses of Heart, so can she be a Princess and her own Gurdian? I thought the Guardians of Light pretty much could be thought of as Knights to the Princesses.
But how would they extract Roxas from Sora though? (Am I missing something here)
