Kingdom Hearts
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5 Cut Characters - VGFacts Five Trivia Feat. Caddicarus
Kingdom Hearts - Did You Know Gaming? Feat. Yuriofwind
 
Although the lack of Final Fantasy characters in later Kingdom Hearts titles (such as the base game of Kingdom Hearts III) was met with backlash from some KH fans, Nomura found this to be quite bizarre as he always viewed the series as not at all being the "Disney and Final Fantasy crossover" that it is commonly seen as.

"I understand there weren't that many Final Fantasy characters in Kingdom Hearts III. One thing I want to clear up is that a lot of fans are saying that Kingdom Hearts is this collaboration between Disney characters and Final Fantasy characters. But I really feel like that's not the basic concept of Kingdom Hearts; that's not exactly what Kingdom Hearts is.

When we released the first title, we had only a few original Kingdom Hearts characters. When they were interacting with really well-known, beloved Disney characters, I felt nobody really knew these new characters, so it was harder for them to stand their ground just yet. And so, we had a lot of Final Fantasy characters involved to lend a hand for everyone to get to know these [original Kingdom Hearts] characters better.

Now, there are so many original characters from Kingdom Hearts that are so well-loved, and people want to see more of those characters. With Kingdom Hearts III, since we did have so many original Kingdom Hearts characters, it was hard to find room for including more Final Fantasy characters. We're trying to find a good balance for that. I know that some fans were concerned about that and weren't too happy and wanted to see more Final Fantasy characters. That's something we definitely are thinking about. But just 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]..."
Contributed by PirateGoofy
According to Tetsuya Nomura, he said that he did not expect the side characters from 358/2 Days and Birth by Sleep (i.e. Roxas, Xion, Axel, Saix, Terra, Ventus, Aqua, etc.) to be incredibly popular among fans, especially compared to the main reoccurring cast.

"I really didn't think that the characters from these two titles would become this popular. I had thought that players wanted to see more of characters like Sora or Riku. It was kind of unexpected that the characters from these two titles were so well received. [...] So, if possible, it would be great to include more of them or continue to share more of their story in future opportunities."
Contributed by PirateGoofy
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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.
Contributed by PirateGoofy
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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".
Contributed by PirateGoofy
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Outside Merlin's House in Traverse Town in Kingdom Hearts, the likeness of an owl can be found on the broken archway in front of the house as well as in the form of several small statues surrounding the building.

Outside Merlin's House in Hollow Bastion/Radiant Garden in every subsequent game, this owl's presence is reduced to two emblems hanging over Merlin's front door both inside and outside the house.

These owls are a reference to Merlin's pet owl Archimedes, a character from Disney's 1963 film adaptation of the T.H. White novel "The Sword in the Stone", who does not appear in the Kingdom Hearts series.
Contributed by MehDeletingLater
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.
Contributed by PirateGoofy
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.
Contributed by PirateGoofy
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.
Contributed by PirateGoofy