Originally Posted by RocketShadow
Well the guy who created them works for Square, but it wasn't until later in making the game that Disney allowed any Square characters in it at all. So it was going to have Sora, Riku, Kairi and Disney characters only until Disney let them put in Squall, Cloud, Yuffie, etc. Square was mainly there to make the RPG aspect originally.
Also all Heartless are copyright of Disney.
I beleive Ansem, the Unknown Boss from Final Mix, and the cloaked people in Another Side, Another Story/Deep Dive are Disney too but I'm not entirely sure.