How Can a Pure Girl Make a Contract with the Devil?

Suppose, if, what if... I choose the Demon King as my contract partner, will the battle be easier? This is a world where you can contract with otherworldly creatures. Monsters, elves, magical creatures, and even demons can all be contracted! Both parties to the contract live and fight together, become close partners, and work towards becoming a top contract user! This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.