Well, it isn't that complicated. BBS is a prequel to explain the origin of the Keyblade and why Sora has it. 358 is pretty much just filler and not of importance to the main storyline. And the enemy's motivation, Soriku, is to gain control of Kingdom Hearts which either grant them power (Xehanort's heartless) or let come into being (Xemnas), though I'm unsure if I'm correct about Xehanort's heartless.
Kimi wa ne tashika ni ano toki watashi no soba ni ita
Itsudatte itsudatte itsudatte
Sugu yoko de waratteita
Nakushitemo torimodosu kimi wo
I will never leave you







