Alright, this is the stuff I've been rambling about across several threads. I just figured it's time there's a thread where it's all in one place, also in case more comes into the light that will support the theory/theories. There is imo now enough material to solidify the theory, although this by no means will suggest the theory must be true. It could easily be wrong and it's not even like there is 1 specific point with it all. In that sense, I suppose one could argue how solid the theory is but make of it what you want. I found out about something today that just was the last drop, causing me to write this thread.
The theory is about Riku's relationship with Xehanort and how they may be related. It has been indicated that they are related already, for reasons explained in the games, but also for reasons not yet revealed. What I have in mind is a possible bloodline connection, that Riku may be a descendant of Master Xehanort, whether it's father and son or grandfather and grandson or something like it. It doesn't have to be a blood connection. It could also be something more enigmatic along the lines of KH's usual plot twists (Eg Xion-Roxas-Ventus-Sora-Vanitas) etc. Most of the things in this thread surrounds the family connection though, as that is also where it all started.
I first thought of this back when the Birth by Sleep trailers came out, showing Vanitas, and everybody was guessing at who could be hiding behind the mask. One of my guesses was Riku, because of the outfit that was similar to that of Riku in Dark Mode. His height and built along with the weapon was also reminiscent of him. Back then, in the same context, I also took note - although I was aware it could just be coincidence - that Apprentice Xehanort/Ansem, Seeker of Darkness/Xemnas shared Riku's hair-color. Let that be the prelude to the theory outline, starting with the old theory contributions through to the latest.
I. Riku's hair is the same color as that of the various Xehanort incarnations. A hair color only seen with these characters (and Ienzo/Zexion, which also is why it could be coincidence). Riku and Master Xehanort seem to have had the same initial motivations. They both wanted to get off the Island (Destiny Islands) and opened the door and their hearts to darkness. They were drawn to the darkness and sought to captivate it, to utilize it, believing themselves powerful enough to overcome it and control it.
NOTE: They have different eye-colors. I cannot recall any of Xehanort's incarnations having green eyes like Riku. It doesn't directly contrast the theory, but it is worth noting that there's a good chance they would have shared eye-color, were they related by blood, considering it's Kingdom Hearts, but also since it is an on-going game story in common.
II. The Soul Eater, Riku's weapon. The basic designs that went into this weapon are seen in other weapons associated with Xehanort. Especially the eye in the center of the weapons.
The ones I remember noticing having are;
2. Ansem, Seeker of Darkness
3. Master Xehanort
4. No Heart (Including his Wing Glider design)
5. Vanitas/Vanitas Remnant
Like mentioned, they are all related to Xehanort. Ansem SoD was his Heartless, No Heart was Master Xehanort's armor - and a form fought during the battles against Xemnas - and therefore just used the same weapon as Master Xehanort himself. Vanitas and Vanitas Remnant might have been extracted essence of Darkness from Ventus (and to an extent, Sora), but the one who extracted this darkness and gave it form was Master Xehanort. Riku? That's the question. Both his Soul Eater and Way to the Dawn weapons have the design...
Ventus pulls this weapon from the ground in The Gathering, the secret scene for BbS in KHII. If this has much relevance I'm not sure about, but one can of course always relate it to Vanitas who was extracted from Ventus.
NOTE: Another possibility is that it's not really related to Xehanort as such. It could just be a sign of the power of Darkness itself. That Riku took the Dark Mode shape as a result of Ansem SoD's influence might have been a simple symbol of darkness taking over, just like Vanitas was darkness-infested. But Riku got Soul Eater before that happened... And all in all, it is just a big question where the shapes and designs of the appearances and weapons origin from. As I see it, there are 3 possibilities within the limits of reason. 1 is that it's a token of Darkness, another is that it's a sign of Xehanort's Darkness and powers, or lastly, there is a family bond between Riku and Xehanort. For the second option to hold water, option 3 would pretty much need to be true too, unless the bond is not by blood, but by something else (went over this already)
III. In the beginning of Birth by Sleep, there is a short cutscene where we see a young Xehanort at Destiny Islands, looking into the horizon while talking about seeking other worlds and getting off the Island, essentially. So we know Xehanort started here. This was before he was properly introduced to Darkness, other worlds and the Mark of Mastery. Why was Xehanort there? Not only is his speech and mindset the same as Riku's before the end of KHI, but theres the hair again, lulz. 2 characters with long silver hair from the same Island? Hmm... There is another last note about this in one of the last points soon, but I am mentioning it later because it is the thing I just learned today and thus is last in the chronology, lol.
IV. Kingdom Hearts: Re:Chain of Memories talks a lot about Riku's story. When I played the Chain of Memories, the GBA version, things were new and I did not have many games to link them with since there was only KHI before it and KHII wasn't out yet. Even with Re:CoM in Japanese, there wasn't much I took notice of regarding this particular theory... But when I played the english version now after having played Birth by Sleep, I suddenly see some of the things that were said with different eyes.
It is referenced in the game many times by Organization members that Riku has an unknown connection to Xemnas. At first they talk about his scent being the same as his, although that is explained as being due to Ansem SoD residing inside him, giving him Xehanort's Darkness (since Ansem SoD and Xemnas both hail from Master Xehanort, they have the same scent). But even after that, there are hints that there is more to it. Especially a sentence Lexaeus says upon his defeat rings in my mind;
"You are the Superior's-"
Indicating the sentence is unfinished. If he is Xemnas's SOMETHING, it probably doesn't make much sense for the relation to just be the same scent of Darkness. No, Riku is something TO Xemnas. I am so tempted to put "son" there or something like it, lol. But yeah, could be many things. Either way it points towards the theorized connection.
V. Back to the Xehanort at Destiny Islands scene. Before I thought it was a plausible theory, but after what I saw today, I get the feeling there must be SOME connection no matter what. I had the feeling before but now they are making it obvious, despite me not noticing till today.
Anyway, I am talking about the voice of young Master Xehanort as he says his words on that beach. Turns out, David Gallagher is voicing him in that one scene, the same voice actor that voices for Riku. So Young Xehanort sounded like Riku? I can hear it now that I know it, but I didn't notice till I read some stuff about Gallagher on wiki and saw his voice creditedin BbS for Young Xehanort.
So assuming the voice spoken there is supposed to be Xehanort's and not Riku simply narrating the scene to insinuate their similar background (which we can't see as Xehanort has his back turned and thus doesn't show lip movements), I don't think I need to say much more. David's voice is arguably even slightly different in tone as he talks in that scene.
Okay, that's mostly it. During this writing, I also had another stupendous idea which I doubt is true but hey, might as well add it. Super secret point VI
VI. Let's say Riku is Master Xehanort's grandson... Which means Xehanort had a son. Wouldn't there need to be a plot involving the father? Well... We know Master Xehanort plans his return with his Heartless and Nobody destroyed and he talks about having certain methods, in other words, aces in his sleeve to make this happen. While it doesn't have to involve his son, there is the chance he is involved. But that's not what I am thinking about mostly with this.
What I am thinking about is Unknown in Birth by Sleep. Like there aren't enough theories about him. But since we never heard of Riku's parents, it could be that his father always lurked in the shadows or never wanted to show himself to Riku or in public for one reason or more. Unknown was pretty much in and out in BbS and possesed enormous strength and ability, something that could fit both Riku's potential and Xehanort's skills. He knew abilities known by many characters, including Organization XIII characters... I'm however not gonna go all out on this part since it's already described in the Unknown theory thread. My point is that the father could have the role I talked about Unknown perhaps having in KH in the Unknown thread. The characters are all associated and certain characters, including Sora, might have some of their abilities due to him for some reason. Probably not, again, was just something funny that came to mind. I do however like the basic idea of it being his dad, lol. I don't think it is at all, no, lol.
Now, since the Destiny Islands voice thing, I just can't see why others wouldn't catch on to something like the same theory already. I haven't checked wikia for similar theories or evidence, but I have never seen any theory such as this anywhere on the internet before, even though I don't look as much for stuff online as I used to anymore. Just saying that I realize this might all sound stupid if it's already been theorized (if nothing else, since the voice actor thing), but like I said in the beginning, I just want to pile all I remember and gathered it in a thread for reasons explained.
Another thing is that I can so see all of this being related to Sora in some way, and not Riku. I don't see how, but since the characters all share some relation to Sora in one way or another, including Vanitas what with the Soul Eater design, I shouldn't rule out the possibility that it once again turns out to be Sora who is at the core of everything and the reason why characters become who they are or are related for this and that reason. It was the same with Xion and Vanitas. It's always Sora, lol xD.