Both Sora and Riku are such terrible picks for a Square Enix rep. Give me Moogle or Chocobo or don't even bother.
KH might not be my favourite SE franchise, but for Smash Bros. it'd be one of the better choices. It's one of their most successful series, has a strong connection to Nintendo platforms, and an easily distinguishable, consistent lead character; unlike other series of a similar scope such as FF and DQ. Aesthetically it'd fit in perfectly, and Sora's abilities would be trivial to transition into a Smash moveset - it'd also provide an avenue to get some FF and TWEWY cameos in the game.
If dealing with Disney wasn't so much of an imposition, Sora could have easily been on the roster already.