Rikku may not be strong physically and magially but she's still vital because of her Steal and Bribe skills. Essential if you want to customize your equipment to its fullest. Also she's the quickest of all characters and can be used to have her turn be given to slow characters like Auron and Lulu.
Most useless will be Kimahri. I don't hate him wholly. Just his role in the Sphere Grid.
And Bahamut might be useless if not for his Mega Flare. He's slow and his Magic isn't even high.
