Birth by Sleep Confusing, but seemingly correct... I think? **SPOILERS**

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I'd have to say either Terra's nobody, or a kind of combination of both Terra's and Xehanort's nobody.

Just a guess, based on how the nobodies work.

Human being - Theart + Xheart = TbodyTsoul(?)Xheart

TbodyTsoul(?)Xheart - Xheart = TbodyTsoul(?).

 

Spoiler

Ok, well Terra -is- his armor now, so basically his body is no longer his own, so Xehanort's nobody is still only Xehanort's.

 

Spoiler

I'd say because MX takes over Terra, We can still say that Xemnas is Xehenort's nobody.

 

Spoiler

Since they merge, perhaps some of Xehenort's memories are Terras or, because Nobodies remember their previous life, Xemnas remembers he used to be Terra, and therefore refers to aqua as friend.

Also, I beleive Xemnas talks to the armour because like with LS, Aqua's armour contains her memories.