In the past FF main games, from VII to XII, there has been at least one character that is pretty much useless that people just doesn't use that much.

1)FF VII: Cat Saith, Yuffie. well never took them into any serious battle

2)FF VIII: Zell. I know some people will say Selphie or Quistis, but they both had extraordinary limit breaks that were really usefull for me

3)FF IX:Quina Quen. I didn't even bother to level her up and I normally have all my players to the same level expect for the lead characther that cannot be taken off the group

4)FF X: Kimary. I always wonder why he didn't have his own grid path, but I did spend my time to complete the whole grid with him

5)FF XII: Penelo, story wise she was useless, but combat not really because in FF XII you could pretty much made anyone looks strong and all my characters were pretty strong, reached level 94 with all of them.

So for the people who had played FF XIII, is there a useless characther among the group?

I can argue to two of those people (Zell and Quina) but oh well.

OT: Sazh is pretty worthless. He has pretty low stats and isn't even the best at his roles.

i only use lightening vanille and fang... i use vanilla instead hope because she learns the better white magic first and because i hate him and i despise snow and sazh has a stupid fighting animation

They all have their uses actually. I guess it comes down to who you prefer to use. I used Fang + Light + Vanille + Hope. Snow when I needed to, and I never used Sazh. He is good, but I just never needed him.

my two least used characters are snow and sazh, but i wouldn't call them useless... sazh is the first one to get haste, so i used him everytime i could during the story, never in the postgame though ^^ and snow is awesome as sentinel in some of the harder missions.

It all comes down to personal preference and what type of Roles you like to use. They are all good at certain Roles and have there own strengths and weaknesses.

For example: Lightning is a great Commando, but she casts Medic spells slower than Vanille and Hope.