Xion and Kairi are both voiced by Risa Uchida in Japanese and by Hayden Panettiere and Alyson Stoner in English.
Several characters who end up basically alternate versions of one character share voice actors, such as Sora and Vanitas (Miyu Irino and Haley Joel Osment), Roxas and Ventus (Kouki Uchiyama and Jesse Mc Cartney), and Ansem, Seeker of Darkness and Terra-Xehanort (Akio Ohtsuka and Richard Epcar).
In III, Akio Ohtsuka not only reprises Ansem, Seeker of Darkness and Terra-Xehanort, but also voices Master Xehanort in lieu of his late father Chikao Otsuka.
Beam Me Up, Scotty!: A lot of fanart, fan-animation, etc. depict the Keyblade being swung with the teeth facing forward, like an ax, when in the actual games it's swung with the teeth facing backward, like a blunt-force object. A side-effect of this is that Ventus' default Keyblade, Wayward Wind, is often depicted in fanart with the teeth facing the wrong way, which is very noticeable given Wayward Wind's asymmetric design.
Cash-Cow Franchise: How else do you explain the presence of five 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.
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 former is Wayne Allwine; one of the games in the series is his very final Mickey Mouse voice role, having been released posthumously). The latter is very noticeable, as both his English and Japanese voice actors, Chikao Otsuka and Leonard Nimoy, died a month apart in 2015! Not only that, but Nimoy's replacement, Rutger Hauer, died almost 7 months after III came out, leading him to be replaced by Christopher Lloyd. Then in June 2015, Sir Christopher Lee passed away, but Corey Burton had been filling in for Lee since 2010.
Colbert Bump: Pretty much any obscure Disney movie gets a HUGE boost in popularity after being featured in a Kingdom Hearts game.
Disney Adventures, the official magazine for The Walt Disney Company, inexplicably delivered a totally inaccurate blurb on the first game's plot, claiming it was about Disney villains trying to change the endings of their stories so the bad guys win. This was the premise of the slightly older Disney's Villains' Revenge, but doesn't really apply to Kingdom Hearts.
In Italy, Disney's official materials (articles on the Italian Mickey Mouse and their strategy guide) about the first game of the series claim that "the invincible Heartless prince" 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, 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.
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. Luckily, David Gallagher's voice hasn't changed a bit in the almost 20 years the series has gone on, but with Haley, it's the most noticeable when he voices Sora in a game where Sora is de-aged to 14 again since they don't seem intent on casting a younger voice double.
Chain of Memories, originally a mostly unvoiced GBA title, was reworked into a full PS2 game as Re:Chain of Memories in 2007 - after the release of Kingdom Hearts II. Thus, 19-year-old Haley Joel Osment appears playing the same version of the character he voiced at 14 to a rather jarring result, whereas in the original, they just reused his battle grunt & quote voice clips from the first game. Averted with the only other teenaged character; 14-year-old Naminé is played by 15-year-old Meaghan Jette Martin.
Brittany Snow was 20 when she voiced the 15(ish) year old Naminé in Kingdom Hearts II.
The first Kingdom Hearts has the remnants of a Bahamut summon in its code.
KH2 Final Mix has unused models for a Woody and Buzz summon.
Sprites related to a Pinocchio-based world are hidden in Days.
A half-finished Jungle Book world is hiding in Birth by Sleep's files.
Exiled from Continuity: Don't expect to see Deep Jungle, the world based on Tarzan, ever again in any Kingdom Hearts games after the first, even in the form of flashbacks or recaps, as Disney is unable to secure the Tarzan trademarks from the Edgar Rice Burroughs estate.note Tarzan and all elements pertaining to it are in the public domain as the copyrights to the novels have expired, but the trademarks to these elements were retained by the Burroughs estate, thus subjecting them to similar restrictions
Surprisingly, Square Enix was able to produce a HD remaster for the original game and retain Deep Jungle, which would be otherwise impossible without cutting the world out. And since some items relating to the controversial world have appeared in coded, it's possible that it's a sign saying that the trope could be averted one day.
Enforced in the games themselves to a certain degree; Disney has strict rules about how the franchises interact. This leads to both The Stations of the Canon (in that the plots of the levels loosely follow the movies they're based on) and an Alien Non-Interference Clause (in that the natives of each world can't be told about other worlds). The rule of thumb seems to be that if you're tied to a particular movie, you're Locked Out of the Loop, though some characters get a bit of leeway in this, including The Beast being a partner during Hollow Bastion in Kingdom Hearts I, Cloud, Zack and Auron being plot-critical in the Colosseum stages in 1, Birth By Sleep, and 2, and the TRON level being Hollow Bastion's operating system in Kingdom Hearts II.
In a sense, Kingdom Hearts V-Cast was made by Disney Mobile, so while it makes sense that he'd appear in Wonderland's chapter from the Disney side of things, the Alice in Wonderland Caterpillar isn't considered canon because Square-Enix's games don't feature him in Wonderland. Despite this, he appears in the manga adaptation for Days.
Flip-Flop of God: Though some of this is also the fault of Word of Dante, there have been conflicting explanations for why Sora and Roxas are able to dual-wield Keyblades. The current official explanation is that Sora has the ability due to harboring Ventus's heart, while Roxas inherits the ability as Sora's Nobody. This is a direct flip of what fans had previously assumedthat Sora got the power from absorbing Roxas, and Roxas in turn got the power from absorbing Xion.
The HD ReMIX Collections being ported to the Xbox One, despite the fact that III was ported to the system. Due to the collections being put together for the specific purpose of catching people up to speed on the games prior to III, as well as give people the most definitive ways to experience them and absorb the stories in the games, it was incredibly odd that Square hadn't brought them to the system sooner, a thing that left many fans confused. Nomura said that he wasn't against the idea, but they had to gauge interest and wait till after III came out to be able to port them due to wanting to not have III delayed. Then you have Tai Yasue saying in 2017 that they had yet to make a decision on that and Nomura said that they still needed to talk to Microsoft about it in 2018. Luckily, this all got sorted with the announcement that they're coming to the system in 2020.
In an Ultimania interview, Tetsuya Nomura states that Sora's lighter hair color in games post II are because of his connection to Roxas. However, he seems to take this back in games made after Re: Chain of Memories, where an entirely new character model for Sora was created that gives him lighter hair akin to II.
God Never Said That: A popular claim is that that the games prior to III hadn't been announced to be coming to the Xbox One yet is because of a supposed exclusivity deal with Sony for the first two main games to stay on PlayStation. Not only does this not make any sense, seeing as the series has been multiplatform since Chain of Memories, which was originally on the GBA, but no one at Square or Sony has said anything of the sort. The most probable reasons for those games being on PlayStation, and nowhere else, thus far is that the PS2 was the most popular gaming console of the 6th generation and Square has a good working relationship with Sony, as well as Square not having another team available to port the games to the Xbox One while III and several other big projects were in the works.
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. Additionally, only KHII was dubbed in Spanish, and neither entry was dubbed in Italian despite receiving a full localization.
III isn't getting any dubs outside of the English dub subtitled in other languages either. It's been stated that this was mostly due to time reasons with the simultaneous release everywhere with the Japanese version.
Kingdom Hearts was, for a time, infamous for including plot-relevant content in Japan-only Final Mix re-releases. Eventually all this content reached the international audience in the form of the 1.5 HD Remix and 2.5 HD Remix (then 1.5+2.5) compilation disks. This was due to releasing only on the PS2 (and PSP for Birth By Sleep) at the time, when Sony had a policy of not allowing games to be rereleased outside of Japan, or possibly just their country of origin, like the way Kingdom Hearts I and II were, without significant enough changes to the base game to justify it. Additional cutscenes, secret bosses, revamped gameplay elements, and additional difficulties were not deemed enough to justify it to Sony at the time. The HD remixes were literally the first possible way to release the content overseas for Square, same with some of their other games that got redone versions of them at a later date in the PS2 days.
coded was almost an example, being a Japanese mobile phone game. This was slightly less painful than the Final Mixes since it's less important to the overarching plot, but it still has a few important scenes and offers interesting reflections on the series' themes. It was eventually remade as Re:coded for the Nintendo DS and given an international release.
Re:Chain of Memories didn't get released outside of the US or Japan in the PS2 days either (it was almost not even release in America until fans begged hard enough as well) and it wasn't until 1.5 came out in 2013 that international players outside of those countries could play that version of the game.
Christopher Lee voiced DiZ/Ansem the Wise in II and 358/2 Days, but his advanced age and health problems forced him to step down. Corey Burton, just as he did for Lee's character Count Dooku in Star Wars, took on the role for all subsequent appearances. It's impossible to notice a difference, as even Lee had praised Burton's perfect impression of his voice.
John Fiedler, the only other voice actor besides Kathryn Beaumont to have worked with Walt Disney, reprised his role as Piglet for the first game only. After his death, he was succeeded by Travis Oates, whose debut in the role was in Kingdom Hearts II.
After her initial appearance in II, Brittany Snow was replaced by Meaghan Jette Martin for the character of Naminé in all subsequent appearances. Martin even voiced her in the Final Mix-exclusive cutscenes for II in 2014.
Kairi and Xion are an unusual example, due to being characters who have to be Acting for Two. While initially Kairi was voiced by Hayden Panettiere, in 358/2 Days both her and Xion are voiced by Alyson Stoner. 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. However, due to Panettiere both being busy with Nashville and her subsequent health troubles, Stoner returned in the Compilation Rerelease1.5 HD Remix, 0.2, and III. This will likely cement Stoner as their permanent voice actor.
Wayne Allwine, the longstanding voice of Mickey since the late '70s, died in May of 2009, shortly after he finished recording for Days. Birth by Sleep marks the debut of Bret Iwan as Mickey. Its a bit fitting, since Mickey is nearly a decade younger and his voice for the character sounds younger than Allwines.
Eddie Carroll (voice of Jiminy since the mid-70's) died in 2010. Phil Snyder replaced him in subsequent games. Then Snyder quit acting and Jiminy was played by Joe Ochman in 2.5'sRe:coded cinematics.
A similar situation happened for the Genie. Initially played by Robin Williams in 2 of the films, Dan Castellaneta, who'd filled in for him in Return of Jafar and the TV show, played him in I and II. In 2.5, however, he was replaced by Jim Meskimen for the Re:coded cinematics, who was a closer voice-match to Williams than Castellaneta's already-pretty-close take. This might have been part of an overall thing for Disney properties, as Meskimen played the Genie in other Disney-themed games contemporary to 2.5's release.
Tate Donovan reprises his role as Hercules in the second game. Before that, he was voiced by Sean Astin (y'know, Sam)? In Birth by Sleep, Josh Keaton returns to voice the younger Hercules.
Additionally, since Nachi Nozawa passed away long before the third game, Vexens Japanese voice is instead provided by Shigeru Chiba.
Jack Sparrow, played by Johnny Depp in the movies, is voiced by James Arnold Taylor (the voice of Tidus and Ratchet) in Kingdom Hearts II, who does a not-so-convincing impression of Depp because of mold in his throat at the time of recording. When Jack Sparrow returns in Kingdom Hearts III, the role has now been taken over by Jared Butler. This was most likely a similar situation with the Genie, as Butler has already played Jack in Disney Infinity.
Also present to various degrees in the European dubs of the PS2 games. Although Disney managed to bring the original voice actors for their characters wherever they could, there were many of them that just weren't possible, either due to the pass of time, schedule conflicts or budget reasons.
The Production Curse: According to Yoko Shimomura, the chorus portion of the iconic "Destati" is believed to be cursed because every time the developers loaded it into the game, something bad would happen. An example of this was the entire building suffering a power outage.
Japanese: Chikao Ohtsuka (Master Xehanort and Captain Hook) is the late father of Akio Ōtsuka (Ansem). Incidentally, Akio Ohtsuka took over the role of Master Xehanort in Kingdom Hearts III after his father passed away.
English: Shanelle Gray (Larxene) is the older sister of Ariel Winter (young Kairi).
Reclusive Artist: Despite having had several roles as a child actor and young voice actor in the past, from Kingdom Hearts II and onwards, Ryan O'Donohue has apparently done little else in the field of acting aside from voicing Demyx and scant details of his current life exist. There don't even appear to any sort of contemporary photographs of the the man, only black and white child photos.
Screwed by the Lawyers: Deep Jungle has never reappeared in any game after the first, probably never will, and just about any references to it have been essentially forgotten thanks to a dispute with the Edgar Rice Burroughs estate (Who still own the trademark rights to Tarzan). The remakes have maintained the Deep Jungle level, but only of the first game.
Urban Legend of Zelda: One of the more common things you'll read online as to why the 1.5,2.5, and 2.8 HD collections weren't ported off the PS3 and PS4 for the longest time is that the earlier PS2 games had an exclusivity deal with Sony to release on only their consoles that extended to their remasters. Not only does this make no sense, since the rest of the games, minus Birth By Sleep came out on Nintendo's handhelds or mobile devices, but Kingdom Hearts III also released for the Xbox One, at the same time as the PS4. Also, Nomura and Tai Yasue also stated that ports of the collections to Xbox One were also possibilities after III was released, but had yet to do so because the team was focused on III's development to not have it be delayed. This seems to be hearsay from some fans to justify the lack of ports of the earlier games to Xbox, despite there being no evidence to support it. In fact, there's more evidence against it. Safe to say, after X019, if there was an exclusivity deal, same as the 3D Final Fantasy games a year before, it's clearly not in effect anymore if it ever was.