Ariel was originally meant to be a Princess of Heart, but in order for her to be captured by Maleficent, the story portion of Ursula giving her legs would have had to take place first. It was already too late in production to add this, so the producers chose Alice as a Princess of Heart, which garnered fan outrage, due to not staying true to the Disney Princesses. However, after finding out the troublesome production of the game, the fans forgave the game developers.
Sora's role as the hero was originally meant for Mickey Mouse. You will notice that Sora wears large yellow shoes, red shorts and white gloves. Disney wanted Donald Duck to be the main character, but Tetsuya Nomura wanted a human protagonist
It is possible to complete the game without visiting the worlds Halloween Town and Atlantica. It's also possible to leave Monstro without having to complete that level too - as Monstro does not have a keyhole.
If you seal all the world's including Colosseum and Winnie-the-Pooh's book and find all ninety-nine puppies and beat Hades cup a secret ending is unlocked which is a preview of Kingdom Hearts II (2005).
The Pride Lands world was infeasible because an additional program was required to process movement on four legs-a necessity since Sora would become a lion in that world and was cut due to time constraints. It was later added as a playable level in Kingdom Hearts II.
If the player completes Deep Jungle before Wonderland, the scene after sealing the keyhole is different. When Maleficent mentions the Princesses of Heart, Alice is normally shown. However if the player hasn't completed Wonderland yet, Snow White will be shown instead.
Sora, Riku, and Kairi are named after the three primary elements of nature. In Japanese, riku means 'land', kai means 'ocean', and sora means 'sky'. the name Kairi could originate from the Japanese word "hikari", which means light, because she is the only untainted main character. "Hikari" is the Japanese version of "Simple and Clean," though the lyrics have an entirely different meaning.
The six summons in the game are: Simba, Tinkerbell, Genie, Mushu, Bambi and Dumbo. Out of the six, only Tinkerbell and Genie join you automatically. The other four must be learned from the Fairy Godmother.
Tifa from Final Fantasy VII was supposed to be an optional boss in this game but had to be taken out due to time constraints. She was later added to Kingdom Hearts II (2005) as a more developed character.
Final Fantasy IX is the only game in the Final Fantasy series from VII - X that did not have a character appearance in this game. However, a differently dressed (but still very recognizable) Vivi took his place in the spotlight in Kingdom Hearts II (2005) (VG).
Early in the planning stages, Yuffie was not in the script. Her role in the story was originally going to be filled by Rikku from Final Fantasy X. However, since there was already a main character named Riku (pronounced the same as 'Rikku'), she was replaced by Yuffie. Rikku did however appeared in _Kingdom Hearts II (2005) (VG)_ with Yuna and Paine.
The four optional bosses in the game are: Sephiroth, Kurt Zisa, Ice Titan and The Phantom. Originally in the Japanese release only Phantom was the hidden boss. When the game was re-released in Japan as Kingdom Hearts: Final Mix, the three bosses from the NA release were added along with a fifth optional boss known as Unknown (which is based off Organization XIII from the later games to which his true identity became obvious in Kingdom Hearts II).
The Assault Rider Heartless that later appears in the Land of Dragons ('Mulan') World in 'Kingdom Hearts II', was originally going to appear in this game, but was removed because it "didn't fit in" with any of the Worlds that appeared in this game.
Although the character of Vincent Valentine from Final Fantasy VII does not appear in this game himself, elements of his character did make it into the game. Cloud wears a red cape and his left hand appears to have a claw--much like that of Vincent's from VII.
The English version of the game's theme song, "Simple and Clean", uses a different melody for the chorus than the Japanese version. However the original melody can still be heard in the US version during the instrumental played over the credits sequence.
In the original designs, Sora was meant to be ashamed to a lion, because Tetsuya Nomura loves lions. Also tried to introduce the world of The Lion King, but he only could introduce the Simba summoning.
According to Mandy Moore, her character was originally named "Aeris", which is the name the character had in the English version of Final Fantasy VII. When she actually went in to record her part, the name was changed to 'Aerith', which is the character's original Japanese name. From the Japanese transliteration for Earth (Earisu- there being no 'th' in Japanese writing), the previous use of Aeris results form the way it sounds in Japanese ("Air-ris").
In the Final Mix version, when Sora and the group confronts Unknown, the audio lines he and Goofy use are recycled from other scenes from the original Kingdom Hearts English release. The Japanese audio variant of this scene wouldn't exist until the later HD release that was part of Kingdom Hearts HD 1.5 Remix (2013).
For the Final Mix version, all the Heartless from the original game have been recolored. This would also be a trend that would also be used in the later Final Mix versions, with the Heartless in Kingdom Hearts II: Final Mix+ (2007), as well as the Unversed in the Final Mix version of Kingdom Hearts: Birth by Sleep (2010). Notably however the Heartless that were later adapted for Kingdom Hearts: Chain of Memories (2004) would be returned to their original coloring.
The original release of Kingdom Hearts Final Mix (along with the later Final Mix releases for Kingdom Hearts II and Kingdom Hearts: Birth by Sleep) use an updated version of the English releases, which as a result uses the voices from the English version but with Japanese subtitles. Unlike the later releases which has new cutscenes with Japanese voice acting, this is the only Final Mix version not to produce new audio for the new cutscenes (aside from the fact the Unknown cutscenes used spliced English audio from other cutscenes). It should also be noted that the later HD releases on the PS3 in Japan for the Final Mix versions would restore the Japanese audio for the original cutscenes and character sound effects.
The trivia items below may give away important plot points.
King Mickey Mouse only appears at the end of the game when he and Sora seal the Door To Darkness. His role in sealing the Door to Darkness is further revealed in the ending of Kingdom Hearts Birth by Sleep 0.2 -A Fragmentary Passage- which is part of Kingdom Hearts HD 2.8 Final Chapter Prologue (2017).
The Hidden Boss Unknown who only appears in the Final Mix version of this game wears the same cloak as the group Organization XIII who would first appear in Kingdom Hearts: Chain of Memories (2004) (originally introduced as "The Organization"). By Kingdom Hearts II (2005), it becomes obvious that Unknown was the group's leader Xemnas due to the fact that he uses the same moves as Unknown during his first battle with Sora.
The secret ending involves the confrontation between Riku and Roxas which is first fully seen in Kingdom Hearts II (2005) (which is also used later on in Kingdom Hearts: 358/2 Days (2009)). Additionally the new secret ending that can be unlocked in the Final Mix version only, is a much longer version that also features other references to Kingdom Hearts II as well (though many of these scenes do not occur in the actual game itself).