There's really three sides of the Kingdom Hearts fandom, and they all draw in different audiences.
There's the Disney side drawing in, you guessed it, Disney fans. The main draw being the several Disney Worlds and Disney characters, which (while disconnected from the main plot of the series) make up most of the games' content.
Then there's the Square Enix side, drawing in fans of the Final Fantasy series and its characters. These characters are a bit more integrated into the original KH storyline, but don't seem to have as big of a presence in the games as the Disney characters.
Then there are the fans who are there for the original KH characters and the original storyline, as complicated and comfusing as it may be. These fans were either drawn in by the Disney content or FF content, but over time grew fond of the original characters.
There may be some overlap, as most fans are there for the original KH content regardless, but rarely do you see overlap between the Disney side and the FF side. It does happen on occasion, though.