When you collect keys of time (copper, silver, and diamond), heroes can be found in the summoning gate. 
 Each type of key is used for a specific gate.
 - The copper is for normal summons (2 to 3-star heroes)
 - The silver is for intermediate summons (2 to 4-star heroes)
 - The diamond is for advanced summons (3 to 5-star heroes)

 Every ten shards of the same hero you get from the gate will unlock that hero.
 The number of heroes' stars reflects the strength of the hero. The higher the stars, the more powerful the hero.