You can unlock addition characters by playing Adventure Mode. Each character is unlocked simply by passing a certain chapter. There are no star requirements.
Schezo and Feli: Complete Chapter 4.
Draco and Witch: Complete Chapter 5.
Lemres and Dark Prince: Complete Chapter 6.
Ecolo and Ex: Complete Chapter 7.
Atlas is an action-rpg with rogue-like elements where you use your ability to control the ground to fight the enemies and move through procedurally generated worlds.