I tend to just re-use the strongest characters all the time, otherwise I couldn't beat the levels. So by the end of the game when it lists my stats, most of my characters would have 0-1 kills, a handful of others might have 10 kills or so, then my strongest character had 180 kills or something like that.

I always felt bad using her for everything because I wanted the others to level up, but they just die most of the time.
I haven't played the handheld games yet though, so I don't know how different those are.