Im not entirely sure of all the characters that cameo but i beleive there are a fair few. Post the in here if you know any!
To get started i have two, possibly the most obvious as u meet them early on.

- Squall
- Wakka

Put anymore in here! I really wanna know!

Squall
Wakka
Selphie
Cloud
Sephiroth
Aeris/th (dunno what she's called in KH II)
Auron (I... think)
Tidus

Dunno about any others, didn't get too far. Actually, I wouldn't mind a complete list either.

YRP
Tifa

I think that Auron is in Kingdom Hearts II, is rikku in it?

yuna, rikku and paine...

yuna and rikku from x-2...

yes, auron is in kh2...

Yuffie is in the first one. Yuffie Yuffie.

I believe that Rudy Gay is in the 2nd one, and JJ is in the 1st. Randy Moss is also a special boss in the 2nd one. I haven't played the 2nd at all, so I may be wrong at this.