Look for the Mysterious Tome to unleash the Strength of Soul and reach a higher limit!
[Mysterious tome] quest will be accepted automatically once you reach Master Lv. 1, and completing that quest will grant you a Mysterious Tome and open Soul Training.
Opening Soul Training enables you to unlock Mystic Land and once unlocked, you can acquire Soul, which is the Mirror Image of your Character.
Mysterious Tome is populated with Mystic Land of Light, Sand, Ice, and various other Mystic Lands, and you can acquire new Soul each time you open a new Mystic Land.
Go to [Soul Training] > Select a Mystic Land > tap on the [↑] button on the top left to check Soul's options and you can upgrade your soul in that menu.