In 1975, architect-turned entrepreneur Yasuhiro Fukushima founded a publishing company by the name of Eidansha Boshu Service Center. Initially they were a tabloid publisher and later attempted to branch into real estate management. In 1982, they eventually moved to video game publishing and officially changed their name to Enix Corporation. In 1985, a company by the name of Chunsoft began development on Dragon Quest and made agreement with Enix to do the publishing for the game. Key figures that were part of the project (and still are in many ways) were composer Koichi Sugiyama, Dragon Ball creator Akira Toriyama, and game designers Yuji Horii and Koichi Nakamura.
There was an electric power conglomerate in Japan by the name of Den-Yu-Sha (Not to be confused with Denyusha which was a game development company that briefly partnered with Nintendo and The Pokémon Company). Den-Yu-Sha was run by Kuniichi Miyamoto. Kuniichi Miyamoto's son, Masafumi Miyamoto, had little interest in the electrical division of the company and instead became a software developer in the Yokohama branch with a focus on video games. Eventually in 1983, Masafumi transitioned the Software Development division into a company that we now know as Square (or Squaresoft internationally).
Enix was primarily known for publishing Dragon Quest, however they were suffering from financial instability. Dragon Quest saw a lot of success in Japan, but not so much internationally. Square/Squaresoft on the other hand was famous for Final Fantasy (I to XI and X-2), Chrono Trigger, Kingdom Hearts I, and co-developing Super Mario RPG. They had their own fair share of financial troubles, however, and their future was in question. A merger between the two companies was announced in 2002 and the deal was closed officially in April 2003. Thus, Square Enix was born. The first games to be under Square Enix were the Windows version of Final Fantasy XI in Japan, Front Mission 4 (Japan), and the North American versions of Final Fantasy X-2.
Super Mario 64, while not the first ever 3D Platform game, is highly regarded as a title that revolutionalized 3D video games. The success of it led many developers to try and make their own 3D platormer games. This spawned a fateful conversation between two employees of Square/SquareSoft. The story goes that Shinji Hashimoto (a Producer at Square/SquareSoft) and Hironobu Sakaguchi (the creator of Final Fantasy) both conversed about how cool it would be to develop a game like Super Mario 64. Alas, they lamented something along the lines of "it would only be possible with Disney characters or something." Tetsuya Nomura (the now director of the Kingdom Hearts and Final Fantasy franchises) allegedly overheard this conversation. The exact date of this conversation is unknown, but considering that Kingdom Hearts started its development in early 2000, it is speculated to have happened some time in 1999.
At this time, Square shared an office with a Japanese branch of Disney Corp. Some time after the original conversation, Hashimoto encountered a Disney Japan executive in an elevator. During this elevator ride, Hashimoto made a pitch for a 3D Platforming game that was very similar to Super Mario 64, but with Disney characters. Executives from both Disney and Square greenlit the project and Tetsuya Nomura was brought on as the game's director. Nomura then made several changes that all culminated to what we now know as Kingdom Hearts I.
The Kingdom Hearts Intellectual Property (IP) is 100% owned by Disney and is developed/published by Square Enix.
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