Good characters become even better as you upgrade them and equip them with great gear. You get the gear from the treasure chests you can get for free or buy in the store and you can upgrade existing gear to make it even better. Then, you can equip it to your characters and once you complete the full set, you can promote that character so he or she gets better stats. Have in mind that more than one hero can use the same type of equipment, so plan your upgrades accordingly and always start equipping the hero you want to upgrade first.

Also upgrade the skills of your heroes. The Skill level can't be greater than the level of your Hero and the golden rule of thumb is to try and keep those levels maxed at all times. This will also prove to be extremely expensive, so again start with your strongest characters first.