They are also very easy to max. You should get them quickly up as you'll need them on the near last missions.
On normal I got them to max as fast as I could (Only to notice their purpose on the fourth last mission
) and pumped attack and defense depending on characters.
And you shouldn't pump strenght on any other than maybe Eruruw and the archer boys as they don't need to be in the front lines. (They might get 2-hitted in the final mission thought
)
Also, Doriy pwns Guraa by a mile and there's no reason to have both.