Once a Soul has been equipped into your Soul item slot, your Soul-gauge will accumulate a point for every monster that you kill. Once this gauge begins to flash, you can summon the equipped Soul by pressing SELECT. (At this point the soul-gauge will reset. This also happens if you decide to un-equip or change the current item in your soul-slot.) Souls only effect monsters that are currently visible on the screen.

There's only 8 Souls in the game, but each one has five levels of power. (The power of a soul is indicated in roman-numeral after the Soul's name [E.g. Yogurt III is Yogurt's summon at level 3].) Higher leveled Souls require more kills for the soul-gauge but they of course do more damage. The only way to raise the level of a soul is at the Temple of Light.