A YouTube reviewer called Angry Joe will be starting with KH3 and has said that if he doesnt understand the story, he will hold that as a negative against the game.
As well he should. Spinoff games, movies, novels, etc are meant for side stories that, while they may enhance the plot and understanding of the main series, should not be essential to said overall plot. If I go straight from Two Towers into Return of the King and am suddenly lost plot-wise because I didn't watch two straight-to-home-video side movies and listen to an exclusive audio book and play a video game tie-in released on the Atari Lynx, I would be rightfully pissed off and lose interest.
So as far as I know from playing the main entries in the series, Xehanort / Xemnas as they appear in KH 2 are the true main antagonists of the series. If this particular plot point goes any deeper than this without being first revealed in KH 3 (spoiler alert: I already know it does.), then the series has quite simply failed to properly deliver this aspect of the narrative.
Birth By Sleep and DDD aren't side stories though. BBS is KH0. The're just as important as 1 and 2. ---
My sister's dog bit a hole in my Super Mario Land cartridge. It still works though - Skye Reynolds 3DS FC: 3239-5612-0115