Franchise: Kingdom Hearts

According to an artist comment in the Kingdom Hearts Ultimania guidebook, the Final Fantasy characters seen throughout the series are considered parallel universe versions rather than directly originating from their home series. However, as revealed by series creator Tetsuya Nomura in the Kingdom Hearts Series Ultimania Alpha guidebook, there is one exception to this: Auron from Final Fantasy X, who appears in Kingdom Hearts II in the Underworld area of the Olympus Coliseum world. Nomura's explanation for this is that the Underworld is linked to the Farplane from Final Fantasy X, both being gathering places for the dead. He also admitted that while he was initially concerned that Auron's Japanese-style clothes wouldn't fit in the world, he ultimately didn't change much of his design aside from removing his sunglasses and making him younger.

In each Dive to the Heart segment in almost every game in the series, there is a disembodied voice that can only speak through text that guides the characters (and thus the player as well) through the tutorial of each game. The identity of these voices has been unidentified in every game but Kingdom Hearts: Birth by Sleep (where it is obviously child Sora) and the first Kingdom Hearts. For the latter, in the Kingdom Hearts Ultimania, Nomura was asked who the voice is for the opening of the first game and he finally gave it's identity as being none other than King Mickey. However, this interview was done in the early 2000's before most of the sequels were produced, so it's unknown if this explanation was retconned or not.

According to series producer Tetsuya Nomura, one reason that Mickey Mouse's role was limited in the original Kingdom Hearts was because around the same time as its release, Capcom owned the license to use Mickey Mouse for the game Disney's Magical Mirror Starring Mickey Mouse. The developers persisted and were eventually given the go-ahead to add him into the game "as long as [they only had him in] one scene, from far away, as a silhouette, with him waving his hand or something". As such, Mickey only appears briefly and seen from behind near the end of the game, helping Sora and Riku seal Kingdom Hearts itself.

Mickey's perspective of this event also appears in Kingdom Hearts 0.2 Birth by Sleep: A fragmentary passage. The reason that Mickey loses his shirt is to partially keep the continuity of the original Kingdom Hearts that showed Mickey in his classic shirtless look.

In a rare instance of Kingdom Hearts being referenced outside of KH-related Disney media, the Kingdom Key and Sora's hair can be seen within Sweet Pete’s assortment of cartoon props in the 2022 film "Chip & Dale: Rescue Rangers". An interesting detail about this cameo is that Sora's hair appears to have been traced or taken from DeviantArt user kimbolie12's fan art of Sora from Kingdom Hearts III.

Although the lack of Final Fantasy characters in later games in the series (such as the base game of Kingdom Hearts III) was met with backlash from some fans, series creator Tetsuya Nomura found this to be bizarre as he always viewed the series as not at all being the "Disney and Final Fantasy crossover" that it is commonly seen as:


Nomura felt this approach was affected by the unexpected fan popularity for the original side characters from Kingdom Hearts 358/2 Days and Kingdom Hearts Birth by Sleep (i.e. Roxas, Xion, Axel, Saix, Terra, Ventus, Aqua, etc.), especially compared to the series' main cast. He felt they needed to find a way to balance the original side characters with the Final Fantasy characters for future titles to meet both demands, but "with the sheer number of original characters that we have now, it's hard to say what the exact balance is going to be [in future games]..."

In the "A Wonderful Spring of Mickey Mouse" animated special on Disney+, a Keyblade can be found in the box Minnie is holding at the 17:43 mark. The Keyblade featured is actually the Kingdom Key D, the Key that King Mickey wields at the beginning of the Kingdom Hearts series. This is one of the incredibly rare instances of Kingdom Hearts being referenced in non-KH related Disney media.

In early concept art for Kingdom Hearts, the series' recurring Heartless boss Darkside had an extremely different design. This early design had the boss appear as a goo monster with a bulbous elephantine shape, "eye spots", a toothed mouth, and a Halo over its "head".

Sora's Parents are only mentioned or appear very few times in the entire series despite what their son goes through. The first is in the cutscene "The Keyblade" during the first game where the storm of darkness hits Destiny Islands. Sora's mom can be heard shouting to her son to come down for dinner to no avail. Moments later, Riku tells Sora, when standing under the dark portal and tempting Sora to come with him, that they won't see their parents again if they willingly go. Another time is when Sora talks to Donald and Goofy about Namine's disappearance in Castle Oblivion in Kingdom Hearts: Chain of Memories and, influenced by a false memory, says that he thinks his parents knew about her disappearance. The last time is in Kingdom Hearts: Birth By Sleep Spoiler:when Terra visits Destiny Islands and talks to a very young Riku who mentions that his friend's (Sora's) dad took them out on his boat.

The games' real-time action platforming aspect (in contrast to the turn-based combat of previous Square RPGs) came from a desire by series creator Tetsuya Nomura and others at Square of wanting to make a game extremely similar to and as thrilling as Super Mario 64 after they played that game and were enamored by it. However in order to make the concept stand on its own two feet, they decided to use Disney properties (which made Nomura in particular even more excited and obsessed with the project) as they believed Disney was the only thing that could compete with Nintendo and Mario.

As revealed in by an interview with a head of Disney Interactive Studios in 2017, the original characters of Kingdom Hearts, such as Sora, although created by Square Enix, are wholly and entirely owned by Disney and the company can do as they please with them.

During a Q&A session at MomoCon 2015, Greg Weisman revealed that a world based on his animated series Gargoyles was proposed at one point to be included in the Kingdom Hearts series, but was scrapped due to some team members "leaving Disney". This marks the only known instance of a world directly based on a television series by Disney ever being considered for inclusion in the Kingdom Hearts series.

Besides sharing a few visual similarities, the characters Axel (Kingdom Hearts) and Reno (Final Fantasy VII) share the same voice actor in multiple languages. Tetsuya Nomura did this intentionally to experiment with putting a familiar character in a new environment and role. He also stated in an interview that the two are "different people, but subconsciously alike".