This game contains a feature called a Priority System.  Each time a new character joins, you're given several choices for stat priorities.  For example, choosing "STR, HP, DEF" makes it so the character's Strength goes up the most per level, while Defense goes up the least.  Another option, "No Priority", simply means instead of having stats with high and low priority, all stats are given average priority and go up evenly.  It doesn't matter much which priority you choose, but each character is given a default priority to best match his/her skill in battle.