http://onigiri.wikia.com/wiki/Stats#Character_typeYou get stat bonuses the further you level up and on that wiki page you can see how much each stat is boosted according to each class you pick.
The class name is a little different than in the game though.
Strong = Power
Reliable = Defensive
... I think.
Anyways, look at the stat priorities and decide, because it's not just the starting stats that are different with classes.
Generally, Power or Defensive works best, Power giving the most natural hp and weapon affinity(.. I think), Defensive giving a mixture of physical and magical defense.
Cautious and Kind is definitely not recommended though, Daring is an option, giving you a choice to perhaps work on a somewhat less tanky but possibly a higher damage potential build.