Essence

It is said that Beat Monsters are people, or at least the spirits of people. So that makes Essence the spirits of spirits of people.

Sources and Uses
Each Essence is designated with a rarity. Salvage Beat Monsters of that rarity to get Essence. Note that twice evolved monsters are of the rarity above that of the once evolved and base monsters of the same name, which is... nifty. All Rarity Special Beat Monsters cost 3 Essence.
 * Base monsters of a certain rarity give 1-7 Essence.
 * Once evolved monsters of a certain rarity give 2-14 Essence. Always a multiple of 2.
 * Twice evolved monsters of a certain rarity give 4-28 Essence. Always a multiple of 4. (It says 3-21, but we know the truth.)