How can you fly in Kingdom Hearts 2?

Answer 1

You can't fly, but you can glide. When you have Final Form and you level it up a bit, you will be able to glide after you jump.

Answer 2

Yes, you can fly in Kingdom Hearts Re:coded. You have to get the part of your matrix from Castle Oblivion, and in that part of the matrix you can unlock the Glide Ability!