I think Kimahri is an interesting character. While he may seem a bit brutish and quiet, he still has his interesting traits. Like his dedication to protecting Yuna.

As for in battles, if you take him into Rikku's Sphere Grid with a Level 1 Key Sphere and have him learn Steal and Use, he becomes a secondary thief. You can do this as soon as you get the Lvl. 1 Key Sphere and before you get Rikku. He makes a pretty good thief character. Plus in a certain later boss fight, if Kimahri has Steal, he can steal Level 3 Key Spheres from the bosses. It's a great opportunity considering how rare Lvl 3 Key Spheres are.