Your heroes can each be equipped with 6 pieces of gear and each item will add a substantial stat boost to their base stats. Always try to outfit your heroes with as much gear as you can so they are as strong as possible. You can increase the stat boost of the gear you give them even further by enhancing it, this can be done be done using the 'Quick Enhance' button.
Heroes can also be upgraded directly by using Upgrade Pills which can be obtained from the Hero Shop, Travel, Accompany, Treasure Hunt, Mall, Selection, Auto-Battle, and main quest. When the indicated number of Upgrade Pills is consumed the hero will be upgraded. Once you have upgraded a hero 5 times you can Awaken them. This process costs 1,000 Upgrade Pills and 150 Awaken Gems and will grant the another rarity star and a massive increase in stats.
It is advised that you avoid spending your resources on upgrading the blue (common) heroes. Blue heroes are mainly used for the beginning stage and not competent among other heroes.
It is important that you make sure you upgrade whenever you can so you are able to overcome any tough challenges you may encounter.