What are Souls?
When a player acquires a hero they already have through recruit or any other method, the duplicate will be transformed into a Soul.
Souls are necessary materials used to awaken that certain Hero to a higher tier. If you do not need the specific soul, you can salvage them to turn them into Soul gems.
The main difference between Souls and Heroes is that a Hero can work and battle but cannot be salvaged while Souls cannot work or battle, but can be used as material to awaken or be salvaged.
Extra heroes can be broken down into Soul Gems via Hero Salvage, which can be traded for Hero Shards in the Hero Store.
If a Hero has gained EXP or his/her awaken level is above 1, his or her rebirth will return them to default status in order to be salvaged.
Hero rebirth allows the player to return the Hero to level 1 and receive the experience books, magic beans, and awakening materials used on it ( if the Hero used a soul to Awaken the player receives the Soul as well).