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 Kingdom Hearts 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.

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. Character models were updated or replaced, including those of Sora and Ansem, which were replaced with their models from Kingdom Hearts 3D: Dream Drop Distance. Textures were also updated.

Kingdom Hearts Final Mix
Kingdom Hearts Final Mix was remastered in high definition. The Summons menu was moved to the fourth Command Menu slot and the commands that occupied that slot were moved to ; however, still serves to call Donald and Goofy to Sora. The right analog stick controls the camera instead of the and  buttons, similar to Kingdom Hearts II. In addition, cutscenes can now be skipped. The Zero EXP 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. Several models were updated or replaced to match other games, including those of Cinderella, Aurora, and Snow White. All background themes from the game were also remastered.

Kingdom Hearts Re:Chain of Memories
Kingdom Hearts Re:Chain of Memories was remastered in high definition. As the game can no longer connect with Kingdom Hearts II Final Mix, completing all the cinematic scenes and viewing all the Secret Reports, Roxas's Diary, and Character File entries for Kingdom Hearts 358/2 Days is now the new requirement to unlock the previous connectivity bonuses. In addition, 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. In addition, the musical track "Lord of the Castle", the theme for Marluxia's boss fight, was remastered.

Kingdom Hearts 358/2 Days
Kingdom Hearts 358/2 Days was featured as 2 hours and 50 minutes of fully-voiced, high definition cutscenes across 109 "chapters". It also includes Roxas's Diary and the Secret Reports. A new "Character Files" feature was added, profiling the major characters in the game. 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. Several themes were remastered, including the 358/2 Days version of "Dearly Beloved".

Development
During August 2011, Tetsuya Nomura expressed desire to release a high definition re-release of the game though he had yet to confirm such plans. The idea for a high definition re-release preceded plans for Final Fantasy X re-release, and was a focus for two Square Enix programmers for over a year.

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 were either unvoiced or used English voice acting, would use the original Japanese voice acting. 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 25th, 2013. It was later announced that the game would come to North America on September 10th, 2013, Australia on September 12th, 2013, and Europe on September 13th, 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."