Gaining ranks in the game will give you access to new heroes, each with their strengths and weaknesses. Once you have reached the required rank you can unlock these heroes by using gold coins that you will earn from battles, chests or from buying them. To purchase a hero tap on the "select hero" button from the "battle deck" screen and then select the hero you want to buy. You will also have the option to try out any vailable hero in simulated battles for free!