Heroes. Heroes are playable characters that boost your Attack and HP. Most Heroes also have a unique Perk that enhances the gameplay. There are multiple ways to unlock new heroes: 1) use Summon Keys or Gems in the Hero Summon inside the shop, 2) collect hero shards from the elemental game modes (Daily Elemental Events and Elemental Castles), 3) use Guild Coins in the Guild Shop, 4) use Tower Points in the Tower Tournament Shop. You can access your Hero collection from the Loadout view of the game.
Hero Attack Element. All Heroes have an Attack element: Metal
Fire
Poison
Ice
Magic
Lightning
Using a Hero of a certain element provides benefits in some events. For example, Poison Heroes are stronger in the Poison-themed Toxic Tuesday events.
Leveling up Heroes. You can level up Heroes using Crystals of their specific Element and a Coin price. Higher level Heroes have higher Attack and HP. You can find Crystals in the daily events.
Promoting Heroes. Every 10 levels you will need to Promote a Hero to continue leveling up. You need Hero Medals to do this. Promoting Heroes also unlocks promotion bonuses. Some bonuses apply only when using this Hero; some bonuses benefit you always regardless of which Hero you are using. You can find Hero Medals in the daily events.