I think its very simple, I somehow was able to understand this unconciously and it seems a possibility, for those of you who do not wish to see an early spolier then do not read this, but if your willing to, then please be free to. The truth is Terra is Xehanort's Nobody, that is Xemnas, and the Person who is Ansem the Wise' Apprentice, if you watch the Secret Ending of BBS at the part where Terra gets really mad you will notice his eyes suddenly change to Hazel, and thats the same eye colour that Xemnas has and also the Hair Style, is similar except when he is a Nobody is Hair is slightly longer so eventually he becomes him, and if you have played KH2 and read all the Secret Ansem Reports it is mentioned that when he finds this Young Man whose unconcious and when he revives him the only thing he remmebers is the name "Xehanort" which explains how Ansem gives him the name, and then when he betrays Ansem and casts himself into the darkness along with the other 5 Founding Members of Organization XIII, he becomes Xemnas.
Oh and Xehanort's Heartless (Ansem) the Main Antagonist of KH1 is a separate individual and has no connection with Xemnas, he is actually Master Xehanort's Apprentice, the Dark Soldier in the Black and Red Outfit, if you played KH1 do you remember the Black and Purple Outfit Riku wore when he was being possessed by Ansem, its the same thing except the colour code.So to conclude this,Terra is Xemnas, and the Dark Soldier is Xehanort's Heartless from KH1.