Collect 3 mystic Crystals on Lilith Pike to unchain Celeste and unlock the guardian system.
The guardian system provides a direct boost to the battle stats of your main character and the mercenaries, greatly enhancing your battle rating. In addition, your guardian also helps you in battles by releasing guardian skills, which deal massive damage to all enemies.
The guardian system has 6 guardians, each with 10 levels. You need to activate all 10 levels of a guardian before you can unlock the next one. Activating guardian levels requires Stars, which can be acquired from Mystic Planes. The Stars you collect are shown on the top left corner of the screen when you are in a Mystic Plane.