I actually used Kimahri a lot. He definitely needs some work, but if you make him as a second thief along with Rikku then battles go so much quicker, and lines your pockets as well. His overdrives are fairly useless, but then so was Lulu's: I never really got the knack of her 'fury' spells and the highest tier spells had really hard rotations. 'Fury' Ultima gets you like 2-3 half power spells; it hardly seemed worth it.

Kimahri really shines when you get the specialist spheres, like the 'White Magic/Black Magic/Skill' spheres which allows you to learn any ability which your other characters have learnt. 'Friend' spheres allow you to move on to another track after you've learnt the fundamental Steal and Use abilities. I tend to make Kimahri either another Auron or another Lulu - I generally go with another Lulu just to spam weaknesses.

In terms of who I would say is the most useless character in the series, I would have to say Snow from FFXIII. Snow is outshone by Fang in almost every comparable metric: Fang is the better Commando, (eventually) becomes the more well rounded ravager, a very good saboteur and is a pretty good Sentinel in her own right. Heck, she even becomes a good synergist with the -ra variants of Bravery and Faith (though never make Fang a medic). Admittedly, Snow is the better Sentinel and has more health, but only in the most specific and hardest battles would you want to utilize Snow for his damage-soaking abilities.

While we're on the subject, Sazh is fairly objectionable as well. He's great at first, with early access to haste, but he has the lowest combined attack/magic stats in the game, and generally becomes the second best synergist before Hope, who is awesome in all but his health stat. Sazh is a decent commando whose only real trick is his unique take on Blitz, and a sluggish ravager with a constrained choice of spells. He's a decent medic, a decent saboteur and a decent sentinel but he's outclassed in all of those roles. Seriously, the best team in FFXIII is either Lightning/Hope/Fang or Lightning/Vanille/Fang.