Acting for Two: Xion shares her Japanese voice actress with Kairi, and in Days Stoner also plays The Other Darrin for Kairi. However, Kairi's original voice actress, Hayden Panettiere, returned for Birth by Sleep... as Kairi and as Xion, interestingly enough.
All-Star Cast: Though some of them are replaced with The Other Darrin sometimes, the voice acting credits of the series include Haley Joel Osment, David Gallagher, Hayden Panettiere, Jesse McCartney, Billy Zane, Christopher Lee, Leonard Nimoy and Mark Hamill. And that's just the original characters; we're not listing the Disney and Final Fantasy credits where many famous voice actors reprise their original roles, or characters who got famous voice actors for their first vocalized appearance. Those credits include Mandy Moore, James Woods, David Boreanaz, and Dan Castellaneta.
Author Existence Failure / Character Outlives Actor: Several characters have lost their voice actors over the years, but the most prominent ones are Mickey Mouse and Master Xehanort. The latter is very noticeable, as both his English and Japanese voice actors, Chikao Otsuka and Leonard Nimoy, died a month apart in 2015!
Cash Cow Franchise: How else do you explain the presence of three side story games between KH2 and 3, one of which existing just to tie up all the loose ends left by almost every other game? Given the series's enthusiastic fandom and the fact that it's made by both Square Enix and Disney, it's not surprising.
Cowboy Bebop at His Computer: Disney Adventures, the official magazine for The Walt Disney Company, inexplicably delivered a totally inaccurate blurb on the first games plot; it said that the games story was about the Disney villains wanting to change the endings of their stories so the bad guys win. Already done in Disneys Villains Revenge, but firstly, that's not the premise, and second, the story's even deeper and darker than how they described.
In Italy, Disney's official stuff (articles on the Italian Mickey Mouse and their strategy guide) about the first game of the series kept saying that Darkside (the giant Heartless Sora fights at the beginning of the game and later before the final boss) is the Big Bad of the game (even referring at him as "the invincible Heartless prince"), and that the game ends with Sora fighting him in Neverland. In the actual game, Darksides are a generic heartless (note the plural), and while Sora does fight one again later on, it's in the remains of the Destiny Islands, not Neverland. Since you encounter it in the middle of fighting the game's realBig Bad, Ansem, it may have been a simple misunderstanding.
Dawson Casting: Haley Joel Osment, Hayden Panettiere and David Gallagher have become this in the course of production. When the first Kingdom Hearts came out, they were 14, 13 and 17 respectively, playing characters who were 14, 14, and 15 respectively. However, by the time Dream Drop Distance came out, ten years had passed in real life while only two have passed in-universe.
Defictionalization: Master Swordsmith and Renowned Propmaster Tony Swatton made a Real-Life Replica of the Kingdom Key Keyblade after receiving a lot of requests from fans. No joke.
Exiled From Canon: Due to some legal issues, Tarzan's world has been retconed from existence and doesn't appear in any game afterwards, not even in the manga adaptation or the dreams at the beginning of Kingdom Hearts II.
Fan Nickname: Apparently Luxord's is George Michael for the (unintentional?) resemblance.
Flip Flop of God: Though some of this is also the fault of Word of Dante, there's been conflicting reasons for why Sora and Roxas are able to dual-wield Keyblades. The current official explanation is that it's because he has Ven's heart inside him, and Roxas is able to do so because he's Sora's Nobody. This is a direct flip of what fans had previously assumed—that Sora got the power from absorbing Roxas, and Roxas in turn got the power from absorbing Xion.
Megara (Ash) is a Pokémon trainer whose closest friend is Vivi (Pikachu), and is good friends with Gantu (Professor Oak). In Sinnoh, Aqua (Dawn) is a companion, and in Unova, Riku (Cilan) as well, while Sora (Montgomery) is a stern champion with a Throh, and Roxas/Ventus (Cameron) is an annoying rival who somehow manages to win the Unova league. Aladdin (James) has a habit of being present most of the time, too.
Donald's got a role in each film: first, a mysterious fetus-looking creature seeking to destroy all clones, then a water god who chose Ash to bring peace, then a butler to the father of a creepy girl, then a hunter, then a chariot racer, then an ex-Team Magma member, then a professor who named Deoxys, then a legendary hero, then a Pokémon Ranger, then a cowardly baron, then a scientist who's spent five years studying Giratina, then a maniac bent on killing Arceus Himself, then a kidnapper who works for a seers, then a mayor, then a four-legged Musketeer, then the red leader of the Genesect Army, then a blue ninja bent on capturing Diancie.
Not just that, Nate happens to be a Keyblade welder.
Hilarious in Hindsight: The main franchise article starts with "So Disney and Square-Enix walk into a bar…" That's not too far from the truth. Nomura and a Disney employee walked into an elevator, and the rest, as they say, is history.
No Dub for You: The first two PS2 entries were dubbed into multiple European languages, but all of the following games used the English voice track in European regions - including the HD Remix compilations, despite said European dubs existing for the older games included.note To be fair, including said dubs would have been inconsistent with the newer games in the compilations, and the HD version of KHII Final Mix+ includes newly recorded English voice acting for the added cutscenes.
No Export for You: Kingdom Hearts owned this trope (unfortunately). Neither Americans nor Europeans got their hands on the original releases of Kingdom Hearts Final Mix or Kingdom Hearts II Final Mix+, making everyone mad and further mucking up the narrative.
It looked like the latest installment to the series, coded would go the same way until it was announced in E3 2010 that the game would be remade for the DS as Re:coded and released to Western audiences.
Which was quickly countered by Birth by Sleep Final Mix, with Nomura being evasive when asked of a release outside of Japan.
This trope has finally been averted for all the Japanese-only updates, as the 1.5 HD Remix compilation release for PS3 contains Kingdom Hearts Final Mix, along with Re:Chain of Memories and cutscenes of 358/2 Days, and 2.5 HD Remix features Kingdom Hearts II Final Mix, Birth by Sleep Final Mix, and the Re:coded cutscenes.
The Other Darrin: Quite a few. A bad example being Mandy Moore replaced by the absolutely wooden Mena Suvari. A good example would be Lance Bass being replaced by George Newbern (who made it clear that the line is "Descend, Heartless Angel", not "Sin Harvest" as Bass' rushed delivery had many believing, including Dan Birlew, author of the official strategy guide.)
Xion is an unusual example. In the English version of 358/2 Days she is initially voiced by Alyson Stoner, who was The Other Darrin to Hayden Panettiere's Kairi. Panettiere would later return to voice both Kairi and Xion in Birth by Sleep and Dream Drop Distance, essentially making her The Other Darrin to Stoner's Xion. Stoner then returned in the Compilation Re-release1.5 HD Remix. See Acting for Two above.
Sleeper Hit: It can not be understated that the announcement of a Disney/Final Fantasy collaboration was derided by majority of gamers when first announced. Kingdom Hearts was released during a time when Disney Studios was going through a Dork Age, so most gamers felt that the collaboration would bomb. The end result was popular reviews, the game ending up as one of the top 10 selling PS2 titles, and the start of one of Square's biggest franchises.
The game was originally supposed to be Mickey-themed, but there was a disagreement on whether Mickey or Donald should be the protagonist. They eventually decided to Take a Third Option and feature an original character as the protagonist.
Chernabog was to have been the original Big Bad, the embodiment of all darkness, taking inspiration from American Gods with the final battle being a gradual climb up the slopes of Bald Mountain to fight him. He was instead turned into a surprise boss due to console limitations and Executive Meddling.
The final battle with Xemnas in KHII was supposed be quite different, with Xemnas melding into a giant Transformer-esque creature◊ after absorbing power from Kingdom Hearts and, upon defeat, transform into this◊ distorted version. This idea was nixed and replaced with the dragon-fortress/armored Xemnas and black-and-white coat Xemnas battles.
Variants of Behemoths and Wyverns (from the first game) were originally included in the 1000 Heartless Battle in Kingdom Hearts II.
The Pridelands were originally going to be featured in the first game, but the engine couldn't handle the quadrupedal movement, so Simba was included as a summon instead. After changing the game engine, the world managed to make it into Kingdom Hearts II.
358/2 Days had a Pinocchio world which was Dummied Out. It would have included Gepetto, Foulfellow and Gideon, and Human!Pinocchio, and would have a What Measure Is a Non-Human?-centered plot and a focus on circus scenes. It was cut due to space limitations.
A Jungle Book world was Dummied Out of Birth by Sleep, with Louie's Throne Room remaining as part of the game code.
The outside walls of Hollow Bastion/Radiant Garden were originally going to be playable in Kingdom Hearts II.
An odd example is a scene from the Beta trailer of the first game, which has Riku at the top of Big Ben. What's odd is that, despite never appearing in the finished game, it is still referenced in later games as canon.
Both Hades and Megara were set to appear in 358/2 Days.
Pleakley was meant to appear in the Lilo & Stitch world in Birth by Sleep.
Rikku was originally supposed to have been with Leon, but the production team decided it'd be too confusing with Riku and Rikku, so she was replaced with Yuffie, who also got the outfit Rikku would have worn.◊
Vincent Valentine was planned to feature in the first game, but was replaced by Cloud, a more recognizable and iconic character. Like the Rikku-Yuffie example above, Cloud's design in KHI subsequently included elements from Vincent's, such as the red cape and clawed gauntlet.
A younger version of Irvine Kinneas was also supposed to be in Destiny Islands, but was replaced with Wakka (also more recognizable, although perhaps because of Kingdom Hearts).
Both Leon's and Cloud's pasts were going to be touched upon in Birth by Sleep, but as the world already had several members of Organization XIII appearing, it was decided that the world would have been too overloaded with characters.
Marluxia was originally envisioned as a woman, but was changed to a man when the developers realized that having two women rebelling against an otherwise all-male Organization and get killed for it was rife with Unfortunate Implications.
The members of Organization XIII who died in Chain of Memories (Vexen, Lexaeus, Zexion, Larxene, and Marluxia) were going to be bosses in the Olympus Coliseum in KHII, but were cut. The Data-Battles of Final Mix are a remnant of this.
Following the death of Wayne Allwine, Nolan North was considered to succeed him as the voice of Mickey Mouse. However, his busy schedule, particularly with Uncharted 2: Among Thieves, prevented him from getting the role, and Bret Iwan was cast instead.
Around 2002, Disney commissioned a Kingdom Hearts animated series... but it was passed on, despite excellent testing, to avoid contradicting future games and to respect Tetsuya Nomura's vision. Some storyboards can be seen here, and an interview with the show's creator here.