Kingdom Hearts HD 1.5 ReMIX

Kingdom Hearts HD 1.5 ReMIX, stylized as Kingdom Hearts HD I.5 ReMIX, is an HD remaster compilation of the  series, made in celebration of the series's tenth anniversary. It includes full HD remasters of Kingdom Hearts Final Mix and Kingdom Hearts Re:Chain of Memories with added PlayStation 3 Trophy support. It also includes a collection of cutscenes from Kingdom Hearts 358/2 Days up-scaled in full HD within the compilation's Theater Mode. It marks the first time Kingdom Hearts Final Mix was made available outside of Japan, and the first time Kingdom Hearts Re:Chain of Memories was made available in Europe and Australia.

The credit roll viewable from the title screen contains cutscene footage from Kingdom Hearts II Final Mix, Kingdom Hearts Birth by Sleep Final Mix, and Kingdom Hearts Re:coded, hinting at the announcement of Kingdom Hearts HD 2.5 ReMIX.

The collection itself was remastered and released alongside Kingdom Hearts HD 2.5 ReMIX as Kingdom Hearts HD 1.5 + 2.5 ReMIX for the PlayStation 4 in March, 2017, Xbox One in February 2020 and PC via Epic Games Store in March 2021.

Development
During August 2011, Tetsuya Nomura expressed desire to release a high definition re-release of the Kingdom Hearts games, though he had yet to confirm such plans. The idea for a high definition re-release preceded plans for a  re-release, and was a focus for two Square Enix programmers for over a year. Plans for an HD re-release were further hinted in a Secret Message in Kingdom Hearts 3D: Dream Drop Distance, unlocked during the credits.

In September 2012, Square Enix announced Kingdom Hearts HD 1.5 ReMIX. It was revealed that the character models from Kingdom Hearts 3D: Dream Drop Distance, like Sora, Riku, and Ansem's for example, are being used in Kingdom Hearts HD 1.5 ReMIX as a base for the game's characters. The cutscenes from Kingdom Hearts Final Mix, which used English voice acting, would use the original Japanese voice acting, though the Final Mix-exclusive cutscenes would still remain silent for both versions, much like with the original release. The only exception is the cutscene with Unknown, as it used lines recycled from other cutscenes, just like the ones from the English version were used for this cutscene for the original Japanese PS2 release. Every cutscene from Kingdom Hearts 358/2 Days, whether it was originally a prerendered scene or used in-game graphics, would be remade for Theater Mode. Kingdom Hearts 358/2 Days was originally planned to be an HD remade game, however, it would delay the development of the new title, so it ended up being within the compilation's Theater Mode.

In January 2013, Jesse McCartney, the voice of Roxas, posted a picture on Instagram showing Roxas clearly displayed on a screen in a recording booth, along with the caption, "For all you 'Kingdom Hearts' Fans. Recording the next chapter, #KingdomHearts #Roxas #Gamers." McCartney also posted a minute long video of himself recording battle and combat grunts for Roxas, while also filming what looks like Kingdom Hearts 358/2 Days. However, when notified, Square Enix immediately removed both the photo and video. After much fan speculation, a 2013 Q3/Q4 release was confirmed for the west on February 25, 2013. It was later announced that the game would come to North America on September 10, 2013, Australia on September 12, 2013, and Europe on September 13, 2013.

It was revealed in March 2013 in a Famitsu interview, that 66 tracks spanning across all three games (with a majority of them concentrated in Kingdom Hearts Final Mix) have been re-recorded with live instrumentation, as opposed to the original games' use of the PlayStation 2's built-in tone generator for their background music.

In June 2013, Nomura stated that the original game assets for Kingdom Hearts had been lost some time ago. He explained, "[The game data] was lost, so we had to research, and we had to dig out from the actual game what was available and recreate everything for HD. We had to recreate all the graphics and it was actually not that easy."

Changes
Changes to all three games include support for various video outputs (480p, 720p, 1080i, 1080p) and audio output supported in Dolby Digital 5.1 sound and DTS Digital Surround 5.1. There is now PlayStation 3 trophy support and an unlockable PlayStation 3 theme for each title.

Kingdom Hearts Final Mix

 * Kingdom Hearts Final Mix had its graphics and several character models remastered in high definition. For example, Sora, Riku, and Ansem's models have been replaced with those from Kingdom Hearts 3D: Dream Drop Distance. Also, the models for Snow White, Cinderella and Aurora have been replaced with their models from Kingdom Hearts: Birth by Sleep.
 * The soundtrack has been completely re-orchestrated by Yōko Shimomura's original score team and performed by gaQdan.
 * Cutscenes were made skippable.
 * Cutscenes that previously only had English voice acting now have Japanese voice acting as well.
 * The combat system and HUD were revised for ease-of-use with influence from Kingdom Hearts II, with the Summons menu moved to the fourth Command Menu slot, the commands that previously occupied that slot moved to, and the right analog stick given control of the camera instead of the shoulder buttons. However, still serves to call Donald and Goofy to Sora whenever a Reaction Command does not prevent the input.
 * The EXP Zero and Combo Master abilities are now available.
 * The  trailer that occupied the third slot on the game's opening menu is replaced with a Back command to return to the Kingdom Hearts HD 1.5 ReMIX game select menu.

Kingdom Hearts Re:Chain of Memories

 * As the game can no longer connect with Kingdom Hearts II Final Mix, unlocking the Kingdom Hearts 358/2 Days wallpaper for the PlayStation 3 menu by completing all the cinematic scenes and viewing all the Secret Reports, Roxas's Diary, and Character File entries for 358/2 Days is now the new requirement to unlock the previous connectivity bonuses.
 * The Monochrome, Follow the Wind, Hidden Dragon, and Photon Debugger attack cards were replaced with Maverick Flare, Midnight Roar, Total Eclipse, and Two Become One from Kingdom Hearts 358/2 Days.
 * The Enemy Card received from Riku Replica, originally known as the Riku card, has been renamed as the Riku Replica card for the HD version. However, this card is still received immediately before the scene where the character is explained to be a replica, so this change creates an in-game spoiler.
 * The musical track "Lord of the Castle", the theme for Marluxia's boss fight, was rearranged.

Kingdom Hearts 358/2 Days
Kingdom Hearts 358/2 Days was featured as two hours and fifty minutes of fully-voiced, high definition cutscenes across 109 "chapters" with several of the characters who were silent in the original game having new audio dialogue. It also includes Roxas's Diary and the Secret Reports. A new "Character Files" feature was added, profiling the major characters in the game.

While this movie version of the game covers the main and most important Organization XIII interactions, the Disney-focused content had to be summarized into short text so it would be under three hours. Some of the removed non-Disney-focused scenes include:
 * Roxas's first interactions with Hayner, Pence and Olette.
 * Roxas confronting Saïx.
 * Roxas obtaining Oathkeeper and Oblivion.

Originally, there was no scene included regarding Xion's boss fight, causing her to awkwardly collapse immediately after preparing to fight Roxas. A free DLC was later released for Kingdom Hearts HD 1.5 + 2.5 ReMIX on PlayStation 4, which added the scene. This DLC is automatically included in the Xbox One, PC, and Nintendo Switch versions.

Sir Christopher Lee, who originally voiced DiZ, was unable to provide new dialogue, so Corey Burton redubbed all of his lines. The late Wayne Allwine, who originally voiced Mickey Mouse, was retained on the cast list despite Mickey's lines being voiced by Bret Iwan. In addition, several other actors whose roles in the original release were archived sound recordings are also credited, even though their characters do not appear in any of the new cutscenes (such as Haley Joel Osment and Jim Cummings). Several themes were rearranged, including the Kingdom Hearts 358/2 Days rendition of "Dearly Beloved".