With such a large cast, which character tends to stay on the sideline the most?
For me, it would probably me Umaro and Gau.
As for Umaro, I just don't like not being able to control the character. I know he's a "secret" character, but he feels a little too shoehorned in the game, as well. I mean, you could make the same argument about Gogo, but Gogo is a cool, character that can be customized.
Gau would be the other one. I like this character and story, but I just can't ever get into enemy skills (with the exception of FFVII).