You Heroes’ attributes may be increased by getting more experience and advancing their level, but also with upgrading their rarity,
 
Advancing the level is easy: just send them to battle and don’t forget to place them onto the battlefield. You can also upgrade their level with gold. But remember that higher levels are more expensive.
 
The level cap is defined by your Barracks: each level of the Barracks pushes the cap for your Hero by 10. For instance, if your Barracks are level 1, your Hero is capped at 10, and for Barracks level 2, the cap is 20. If you Hero reaches the limit, they will stop getting new experience unless you upgrade the building.
 
Hero rarity may be upgraded with personalized tokens that you can earn by completing Tower of Trials levels.