We don't know much about them but one thing is clear to us now. Spirits were never supposed to walk on this planet as their native abilities allow them to transform entire landscapes by twisting mana itself. As such we did not allow them to cross the border when we tapped their realm. But soon people began to sympathize with them and so they formed pacts with them. They would be limited to only the mana of their partner but in return they were free to walk in our world. Some were grateful and used their incredible strength to assist their new partner. Others sensed the chance to fully overtake their hosts. No matter how we looked at it there was no chance for a compromise.
During your travels, you will encounter many unique and interesting people. Some will reward you with lore, but whether or not you believe these people is another story entirely.