What about Leonard and Brad? Mine have always turned out badly, although I'm sure they can be good with training ... right?
Pretty much.
Leonardo's strong selling points are his skill, luck and resistance, while having rather poor stats everywhere else. Max these 3 stats ASAP while gaining some stats in the other departments and you'll have a good unit, especially since his third tier class is really good.
Brad is actually good in the first place as the Dawn Brigade's tank. His strong points are strength, skill and defense so once those are maxed you can dump some bonus experience into him.
As a side note, if your character has only one stat to cap left, I think you'll always get the stat in question until it's capped. I never ever had a no stat level up in this game, although perhaps it was just the difficulty. Does anybody had a no stat level up unless all your stats were already capped?