Larxene shares many similarities to Asuka, basically sharing her bitchy attitude but with fewer redeeming qualities and an even more damaged psychosis. Tellingly, Nomura said that Larxene's voice was the first one cast for RE:Chain of Memories and that Yuko Miyamura was always his personal choice.
Dawson Casting: In the remake, Sora is supposed to be fourteen at this point like in the original Kingdom Hearts, yet the voice of Haley Joel Osment sounds nothing like a kid anymore. Averted in the original Game Boy Advance, however, because Sora's voice came from sound files reused from Kingdom Hearts.
Fan Nickname: The Riku Replica is occasionally referred to as "Repliku". It's even his official name in the D-Report in the German version.
Flip-Flop of God: Nomura stated in an interview that Nobodies do not age (with the exceptions of Roxas and Naminé). However, Birth by Sleep and Dream Drop Distance appear to contradict this with the ages of Braig and Ienzo, though it is revealed that Xigbar was not going grey because of his age, leaving it as a possible case of Immortality Begins at 20.
The Diamond Dust and One Winged Angel keyblades are in this game. They actually came from Final Mix, which nobody outside of Japan saw without importing it (at least until 2013 and the 1.5 HD Remix).
The Neoshadows, who only showed up in the original game in the secret ending, but were limited to a secret cutscene, and similarly to the above Keyblades were introduced as actual enemies in Final Mix. They return in this game.
Deep Jungle was intended to have made a return, but Disney lost the copyright to Tarzan so it was axed. All that's left in the game is the card for the King of the Jungle Keyblade, albeit Dummied Out.
The Dive to the Heart was planned at some point. It presumably would've served as the gameplay tutorial, which is instead a normal conversation and fight in Traverse Town.
Marluxia was originally planned to be female, hence his pink hair and feminine appearance. It was decided that having the only 2 female members of the Organization conspire to forcibly overtake it had too many Unfortunate Implications, so the gender was changed.