I watched the Gametrailers retrospective, and I have some questions on rules of the Kingdom Hearts universe. I never played 358 or DDD, so sorry if these are dumb.
Okay here we go. Quite a few spoilers ahead so skip this post if you don't want to be spoiled.
So there should be 7 light keyblade wielders and 13 dark keyblade wielders, I take it?
Not exactly. There's seven warriors of light whom posses keyblades. The 13 Seekers of Darkness don't all wield key blades but some of them do. It's not a requirement to have a Keyblade to be one of the 13 darknesses.
Who are the 7 light holders? Riku, Kairi, Mickey, Lea, Ventus, ?, ?
This is something fans are trying to find out too. It's been made vague on purpose. It's supposed to promote discussion on the topic. My theory is that Sora, Riku, Kairi, Mickey, Aqua, Ventus and Lea are the 7 Wielders of Light.
Who are the 13 dark holders? Resurrected Master Xehanort, Venitus, ...?
This is also not been mentioned probably because they want to roll out these 13 bosses throughout KH3. But so far we know that: Xehanort, Young Xehanort, Ansem(Seeker of Darkness), Xemnas, Braig and Isa are all part of the "new" Organization XIII and part of the 13 Seekers of Darkness.
If Org 13 was meant to be the dark wielders, why don't they have keyblades? Even Roxas only used Sora/Riku's.
Explained above.
One keyblade can obviously take multiple forms. Can it be in multiple instances simultaneously? i.e. can Riku and Sora both wield Riku's blade? Are Aqua and Terra both keyblade-less now?
Are you asking if Sora and Riku can hold the same Keyblade at the same time in different areas? If that's what your asking then no. Sora can use Rikus Keyblade and vice-versa but there can't be two Kingdom Key(blade)s being used. Each Keyblade is unique. They can't be magicked into being in two places at once.
As for Aqua and Terra. They still have their keyblades. Just because they both performed the rite of succession on Riku and Kairi doesn't mean they can't still wield their Keyblades. If that was the case then Master Eraqus and Xehanort wouldn't have their keyblades.
Was Venitas given a keyblade by Master Xehanort? Does he just have all 13 dark ones in his pockets?
One would assume. Usually one has to have the rite of succession performed in order to obtain a Keyblade. This is why Sora is so "special" because he obtained a Keyblade without the ceremony ever performed on him.
What other techniques are there for gaining a keyblade? Sora got Riku's through purity of heart; did he "steal" Riku's or conjure up a new one? If the latter, is this one of the 7 or a new keyblade altogether?
The Keyblade Sora has can lock and unlock any door or chest. Lame I know but it explains why you can open up everything in the game.
Every Keyblade is special with what it does.
He didn't steal the Keyblade per-say. The Keyblade sensed the darkness in Riku's heart and moved to the nearest pure heart which at that point in the story was Sora. At that point Sora became the (that particular) Keyblade's master.
Everyone has a unique Keyblade. The Keyblade doesn't make a wielder one of the 7 lights. It just lets them fulfill the basic requirement of being one of the 7 lights.
Are these 20 keyblades all parts of the X blade?
I think you are placing too much importance on the Keyblades each wielder holds. It's when an equal strength of Darkness and Light clash is when the X-Blade is forged.
What happened to all the keyblades that belonged to masters fighting in the Keyblade War? Were these just not passed on, thus only the X-derived keyblades remain?
They are all in the Keyblade Graveyard. And if you have your own Keyblade why would you want to pick up an old Keyblade when you have a perfectly good one?
There are no X-blade derived Keyblades. I don't know where you read or thought that but that's not a thing.
Does the 20 keyblades rule hold over time travel? What are the implications of 12 new keyblade-wielding Xehanorts showing up? Do they all just share Xehanort prime's keyblade?
Like I said before your placing too much emphasis on the Keyblades. Keyblades are just very useful weapons within the world of Kingdom Hearts. Beyond that they hold very little importance. Now the X-blade can unlock and open Kingdom Hearts. That's why such great emphasis is on that particular weapon. Plus it packs a punch and looks awesome.
What are the various techniques of opening Kingdom Hearts? So far, we've seen:
1) single keyblade (KH1)
2) princesses of heart (KH2)
3) use the X-blade, which can be created with clash of 1 light/1 dark keyblade wielder (BBS)
All ended in failure and some of them are "fake" versions of Kingdom Hearts. We haven't actually seen Kingdom Hearts yet. So we assume that the proper method hasn't been found but this new plan that Xehanort has made might actually open Kingdom Hearts this time. Guess we'll find out in KH3.
What stops Master Xehanort from using techniques 1 or 3 again? Why does he need to have 20 keyblade wielders clash this time?
Because the other times it wasn't the "real" Kingdom Hearts or it ended in failure. Again it's not 20 Keyblade wielders. It's 7 lights, 13 darknesses.
Was Venitus a heartless? If so, did Ventus become a nobody when Venitus was split, or is the Venitus/Ventus split a unique thing?
Ven was never a heartless nor did he ever produce one. Xehanort took the darkness from Ven's heart and gave it form in the form of Vanitus. This weakened Ven and is why when Sora's heart on its way to being born saw Ven's fractured heart and merged with it for a while so Ven could recover. When Ven recovered Sora's heart went on to be born inside Sora. This is why Vanitus looks like an evil Sora. It's also the reason why Ven's heart when it was fractured again at the end of BbS was able to find Sora's heart when he was little and took refuge within Sora and is also the reason why Sora's nobody Roxas looks like Ven.
So in short, it was a unique... experiment done by Xehanort on his pupil.
Are the powers of Namine the result of a rule, or is she just a unique instantiation that's unjustified?
Namine can affect Sora's memories because she is the nobody of a Princess of Light (Kairi) and thus has special abilities not usually shown from a nobody and because of Kairi's special connection to Sora and because she cares about Sora so much Namine has the ability to alter Sora's memories.
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Whew. I hope that helped answer some of your questions.
