AN: This chapter is not exactly story, so unless you want to know the background of how I wrote the story, and certain lore pieces I had to create to get it to work, please skip to the next chapter if you're reading this after I posted it.
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Will
Enemies: Though in the final draft of this story I ended up using the Replicas as my enemies, I had originally planned for the primary antagonists of the story to be the Organization come back for more, but after replaying 3D, I decided that I'd go a different direction. My first thought was creating new Heatless for them to battle that were based entirely off of Darkness, and not at all artificial, but this idea lacked character for anyone besides the heroes, so I decided against it, eventually deciding on the Replicas as the final opponents.
Xion/Oni: Xion was always an important part of the story, but originally Roxitas and Lea both had perfect memories of her for reasons I could never figure out, which is why I never went wight that idea. Xion was also originally going to be a lackey for the Organization for a while, dressed like Vanitas of course, however that would have changed after a fight with Roxitas. After I scrapped the idea of the Organization however I needed something different for her to do, so I put her in the realm between with Sora to help guide him through it. Now anyone who's reading this is probably wondering about the other Oni, the one who looked like Riku. Well he was kind of a combination of some characters i'd been working on for the second two installments in this series before I decided to cut it into one, and then I made him connected to Xion by being her dark half. Then there's Oni's name, the last piece of trivia I can think of about her. Oni is the Japanes word for ogre or Demon, and being a westerner, when I think Demon I think fallen angel, and that seems perfect for Xion's new name, and the letters work.
Roxitas: the idea of Roxitas was a focal point of the entire story sense conception, simply because I wanted some way to redeem Vanitas as a character and make him part of the Keyblade family. Roxitas however developed into a kind of contradictory character that had trouble showing emotion because I was always imagining Roxas and Vanitas arguing about what to say or do, which is why I eventually came up with the idea of them trading out who was in control. A lot of you probably noticed how Roxitas was cold towards Namine and Lea, well that's simply because he was focused on Xion, and Vanitas didn't exactly have too many positive feelings for people, making it hard for Roxas to express himself through Roxitas. After the split Vanitas gained the ability to dual wield his two Voidgears (white and red) and Roxas got back Oblivion Key and Oathkeeper, though he can wield Two Become One on it's own as well. Roxitas is the result of typing in Roxas and Vanitas into a couple name generator.
Dual Wielding: Roxas, Oni, Vanitas, Sora, and Ventus can all naturally wield and summon two Keyblades, however only Oni and Roxas can combine their two Keyblades into one Keyblade. Roxas also has the ability to wield other peoples Keyblades in a way that doesn't allow them to summon them back, but only if they're touching him.
Combat: Roxas is the fastest Keyblade wielder in the new Organization, and can beat Vanitas in a race even if he's teleporting. Sora has the most defensive capability, and is near the top on speed. Vanitas is one of the most agile members, and is impressive at magic. Oni combines magic and combat more fluently than anyone else. Ventus is the most agile, able to move higher and faster than Vanitas, without teleporting or using magic. Aqua is the most proficient magic user. Terra is the strongest of the group, and can take the most hits. Lea is the most accurate magic user, and one of the fastest fighters. Kairi is extremely good at healing spells, even exceeding Aqua. Namine fights in a similar fashion to fencing. Riku combines agility and strength into a deadly combo effect.
Trilogy?: As i've said before, I originally planned a trilogy, but I realized I wouldn't have the time. Originally the first story would have ended with Roxitas and Sora in the real world and Ven and Oni stuck in the Realm between, making the next one about getting them back. The second one would have them find them, but Roxitas had given into the Darkness and was about to turn entirely into Vanitas when Ven pulled Roxas's heart out of Roxitas and into his own body, leaving it himself. The final one involved the Ixion and Ixvion (the sixth Replica who was based off of Roxitas, and looked like Ven) and ended with Vens heart entering Ixvion's body and Ixion becoming good.
Sydney: Sydney was Ven's sister in Twilight Town, and her death at the hands of Heartless is what led Ventus to summon his Keyblade and Master Xehanort to take interest in him.