As you progress in Battle Breakers you will unlock heroes, they will be rewarded for completing specific stages in the game, reaching specific levels or as various daily rewards. The easiest but most expensive way to unlock heroes is by visiting the in-app Shop and spending diamonds. Heroes can be gained as rewards by purchasing a a Premium Pass, participating in the special events, and from the Skybreaker which will reward you with heroes when you reach certain milestones on each map.