Hero Enhancement
Heroes can be enhanced to improve their Attack and Defense. You can find this option in the Hero profile window.
There are 3 different types of Hero Enhance, with different costs and different bonuses. Copper Coin enhances also have a lower Maximum Enhancement than Gold Coin enhances. The maximum enhancement value will depend on your Hero's Star Level and Rebirth Tier (which can be increased via Hero Rebirth).
Copper Coin enhancements cost 500 Copper Coins per refresh. Each refresh will randomly generate a value between -20 and +20. You can enhance your Hero as many times as you like until it reaches this maximum. If you are not happy with the value that was generated, you can choose not to apply it. To apply the Enhance, simply tap the "Enhance" button.