How does one even grow more mature when one is not even concious? I mean physicaly, yeah sure, but don't you actually have to... you know... live and do stuff, see the world, learn, to grow mature mentally?
I actually always wondered why the organisation wanted Xion to be the winner. I mean sure, you want a complete keyblade master, so you want to merge Xion and Roxas. One has to destroy and absorb the other. But why preffer Xion over Roxas?
And now I think I know why. It's because Roxas is a nobody, making him pretty much a human with no heart. He still has a mind of his own. He could easely go against the Organisation's grand designs, like he did. Xion, on the other hand, is a puppet. A product of the replica program. She can easely be reprogrammed, making her much easyer to controll. This is shown too. Xemnas reprogrammed her, and Xion then attacked Roxas, something she would never do otherwise.
The part of Terra remaining in Xemnas is looking for his friends. He allready found something of Aqua and is pretty content with spending lots and lots of time alone in a room talking to her armor. Now he wants to find something of Ventus. Now the question is, how does he know where to find this? Why is he looking in CO?
I guess this could be answered with this:
Xemnas is a bad guy, and bad guys always just know way too much... (it's true. It happens all the time, in many stories. Not only in KH.)
But this time, I just won't be content with that answer.