According to a comment in the Kingdom Hearts Ultimania guidebook, Aya Brea from Parasite Eve was originally planned to appear in the game as a member of Leon's group. Partway through development, however, Aya was replaced with Aerith Gainsborough from Final Fantasy VII due to requests from staff members who had worked on Final Fantasy VII.

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 Kingdom Hearts, the title card for Halloween Town contains a faster, pitched up version of Kefka's laugh from Final Fantasy VI. This detail reappears in Kingdom Hearts II, although it is slowed and pitched down slightly to be easier to make out.

In the original version of Kingdom Hearts on PlayStation 2, Selphie's underwear can be seen as she is sitting down. This was removed in Kingdom Hearts HD 1.5 Remix. However, a glimpse of Jane's underwear can still be seen through an opening on the side her skirt.

Rikku was going to have a major role in the first game. She was going to be a part of Leon's group in Traverse Town and help Sora throughout the game. She was removed because of the similarly-named character Riku, and was replaced with Yuffie. She later appeared with the rest of the Gullwings in Kingdom Hearts II.

According to series creator Tetsuya Nomura, the original data and assets for Kingdom Hearts had been lost, forcing Kingdom Hearts HD 1.5 Remix to be remade from the ground up:

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.

Kathryn Beaumont, the original voice of Alice in Alice in Wonderland and Wendy in Peter Pan, reprised her role as Alice and Wendy in Kingdom Hearts at the age of 64.

Early concept art shows that Sora would have originally donned Aladdin-inspired shoes and a more street rat-inspired version of his usual wear when he was in Agrabah, but in the released version he just wears his normal outfit instead.

A Station of Awakening for Alice from Wonderland was originally created and planned to appear in this game and accompany the Belle, Cinderella, Aurora, and Snow-White ones but ended up never being used. Spoiler:With a possible explanation being the fact that Alice, like Kairi and Jasmine and unlike the other Princesses of Heart, was not abducted by the Disney villains at the very start of the game.

Early screenshots of the game showed Sora with different clothes that were orange and black and a different hairstyle that was black but with large white tufts sticking out the front of it. This early design for the character also resembles the earlier "Lion Sora" design, albeit without the lion elements.

If you defeat Maleficent in Hollow Bastion before chasing Pinocchio in Monstro, a set of alternate cutscenes will play out as you progress through the rest of Monstro. The cutscenes where Riku talks to Sora in the first chamber of Monstro, and the cutscene where he talks to Maleficent are both edited out of this sequence.

In Huey, Dewey, and Louie's Item Shop in Traverse Town, there is a poster of a steamboat with the caption "RIVER BOAT" in a large cartoony font, possibly as a reference to the 1928 Mickey Mouse short film Steamboat Willie. The art style of the late 1920's black and white Disney cartoons as a whole would be adapted as an explorable area in Kingdom Hearts II called Timeless River.

Sora, Kairi and Riku's ages at the chronological time of this game's story are all stated in the PlayStation 2 release's instruction manual. Specifically, Sora and Kairi are both 14, and Riku is 15.

Kingdom Hearts' international release is the last game by Squaresoft released outside of Japan to have their logo and name adorned on its case before the company merged with Enix to become Square Enix in 2003. The Japanese version of Final Fantasy X-2 was last game ever to feature the Squaresoft logo in any region.

Concept art featured in early design documents of the first Kingdom Hearts game shows a world clearly based on Disney-Pixar's Toy Story franchise that, for reasons that have not officially been stated, does not appear in the final release. A Toy Story world would, however, appear in Kingdom Hearts III

During "Dissidia Final Fantasy NT: Behind the English voice cast" with Steve Burton (Cloud Strife's English voice actor), Steve explained the reason why he got the role for Cloud, was because of a guy from "Square" who's in charged for the voices of the characters, was watching a movie of him in "The Last Castle".

A number of Gummi Block Blueprints use the names of characters and enemies from the Final Fantasy series.

Original Kingdom Hearts design documents reveal many differences between early game concepts to those that appear in the final game. These include the appearance of a strange icon depicting a drastically different Destiny Islands, unused Heartless designs, and Sora having clothes similar to Tarzan in Deep Jungle.

After the release of the first Kingdom Hearts game, Disney was planning on making an animated series based on the game. A full-color, voice-acted animatic pilot for the show was produced and received well, but ultimately the show was never made. Since Kingdom Hearts: Chain of Memories and Kingdom Hearts II were formally announced around the time the pilot was made, Disney opted for Square-Enix to continue the story with more games rather than through an in-house animated series.

In 2022, the animatic pilot was uploaded online by its director Seth Kearsley.