In the beginning each Mamodo will start with one spell (Offensive), then as the rp goes along they will unlock new spells (We're figuring out a way to structure this right now). Each Mamodo will have a specific theme of power, like Zatch uses lightning and electricity spells, so usually it will revolve around that.

As for power of spells... They get stronger as each spell unlocks. Spell three may just be a more power version of spell one. So long as you stay in that theme though, you're good.