Have you actually played the games, or just read a plot summary?
Roxas is Sora's nobody, made from when Sora became a heartless, but then got a new heart instead of his old one back. They merge back into one being at the end of Kingdom Hearts 2.
Aqua, Ven and Terra have been seen watching a young Sora and Riku playing, in a preview of Birth by Sleep. That means they are in the wrong time frames to be the same people.
Xion was an experiment; Xemnas wanted a Sora that wasn't Sora, so Sora would never wake up (See Chain of Memories), so he had her out of Sora's memories, but as Sora's memories had been tampered with, she was imperfect, and took the image of Kiari.
So in short, you just sprouted about two paragraphs worth of bull shit.