Different characters fill different roles. A sword indicates an attacker, a shield indicates a tank or defender, a wand indicates a magic user, and a cross indicates a healer. Add a good mix of characters into your party for a balanced attack and defense. Without a tank you'll lose health a lot faster, without a healer you'll die more quickly in tough battles, without an attacker you'll have to tough it out for longer due to not being able to deal much damage, and without a magic user you'll lose out on a lot of splash attacks and support spells.