I refuse to acknowledge Advent Children characters, because it was utter pants. Should be kept to the original game, with maybe an exception for crisis core but that's about it.

Other than the main party + Sephiroth I'd have:

Mukki
Don Corneo
Big Bro
Reno
Rude
Elena
Bugenhagen
Rufus
Palmer


And then maybe bonus characters like the Chocobo Sage and Zack. And Mr. Dolphin