WARNING KH III SPOILERS
I didn’t finish game yet but I want to voice some opinions that’ll help others be less confused.
I think I understand the membership of the real Organization XIII now.
I’ll divide my explanation in categories:
Present Somebodies: Somebodies or recompleted Humans that exist in the present and doesn’t need a Replica Vessel. Master Xehanort and Terra fall to that category. Because Ansem and Xemnas are the Heartless and Nobody of Terranort the Fusion of Master Xehanort and Terra’s Hearts in Terra’s Body. When Xemnas and Ansem were destroyed, both Terra and Master Xehanort was revived, and because they existed in the present they didn’t need a Replica Vessel. Terra however was given a fragment of Xehanort’s Heart to become Terranort again and to be an Organization member. When Sora talked to Terra who existed as the Guardian controlled by Terranort, he freed himself from the fragment’s control and exorcised it to be free again
Past Somebodies: Young Xehanort and perhaps Vanitas fit that category. Young Xehanort is a completed person, but is from the past. He can only travel through time from the point he began his journey to when Master Xehanort was revived. In addition he’s not allowed to interfere in events.
So after Master Xehanort was revived, Young Xehanort was meant to return to his present and forget everything, and grow up into being Master Xehanort, but he was given an extension by being given a Replica vessel to act as both a physical body and a tether that connects him to the present.
As for Vanitas it’s a mystery still. Either the Vanitas we saw was his present self and doesn’t need a Replica vessel or he’s from the past to perhaps before his defeat by Ventus, and was given a Replica vessel to exist in the present as well. If so both Young Xehanort and Vanitas are members entrusted in being Somebodies requiring less control because they’re the most trusted. However it’s possible Vanitas was given a fragment of Xehanort’s Heart which compelled him to obey and be a member of the Real Organization.
Nobodies: my first impression of the former members of the Nobody Organization XIII, was that they’re like Lea, recompleted humans that were given dark powers and fragments of Xehanort’s Heart. But turned out I was wrong, they’re Nobodies from the past, gathered by Young Xehanort and sent to the present to be tied to Replica vessels to exist in the Present.
As long as they exist in vessels they won’t be recompleted humans in the present as they’re likely taken at the moments of their defeats before they’re recompleted, save for Master Xehanort who was completed before Xemnas and Ansem were given vessels.
Or perhaps more accurately they were taken before they were recompleted, like Even. He was asleep before he disappeared and reappeared as Vexen again. That could mean that Young Xehanort came to him before he was recompleted and took his Nobody and remade him into a Replica with a fragment of Xehanort’s Heart to compel him to obey and make him a reserve Organization member. That theory could support that the presence of important people like Xion doesn’t alter current events more.
The Nobodies were recruited because they’re empty husks, easier to control in Replica bodies and given fragments of Xehanort’s Heart which gave them their yellow eyes.
Each were compelled to obey as they’re influenced by Xehanort’s Heart fragment despite their personal opinions and reasons.
When Sora defeated each of them, they became free from Xehanort’s influence and was able to voice what they’re really feel, before their Replica vessel fades away and they were sent back to the past to finally be recompleted as humans in the present .
Replicas: Namely the Riku Replica and Xion. The Riku Replica was taken before his defeat by Riku, who redeemed him and had him fuse his spirit with Riku. But the Riku Replica both motivated to become “real” and compelled by Xehanort’s Heart fragment served the Organization.
As for Xion, I feel she’s a case of affirmative action in response to the failure to recruit Sora as the 13th member.
Xion in the present does not exist as an individual anymore. She’s born from the stolen memories of Kairi. She was meant to copy and absorb Roxas and Sora’s powers to be a Keyblade Wielder’s Under the Organization’s full control. Instead Roxas absorbed her, creating his second Keyblade as it was once her’s.
Sora couldn’t wake up because Xion exists holding the most important memory needed to complete him. So when Roxas rejoined with Sora, Xion’s essence became memory again. In the present she can’t exist anymore unlike Roxas, and the Riku Replica who was made from combat data of Riku without raking anything from him.
In order for her to exist in the present, a past Timeline version for when she still existed as an individual was summoned and bound to a Replica. She was compelled to obey due to having a Xehanort fragment inside her. Perhaps having yellow eyes under her hood. But when Sora talked to her she broke free from the fragment’s control, maybe in exorcised it from herself, being able to exist as her own person ,with no consequence to Sora.
i also speculate the reason why Demyx and Vexen were benched as reserves, was because Demyx is a non-combatant and Vexen is too important to risk in battle as he can create new replacement Replica more efficiently.
However it’s also because perhaps they’re the least trusted as it seemed they’re the least influenced by Xehanort’s Heart fragment.