Following that line of logic, Kain re-read most of the notes about Series X in a new light.
He came to a conclusion, the reason why humans can only form a contract with a limited number of spiritual creatures, known as their affinity, seems to be due to some kind of mysterious restriction. Something that this mysterious organization refers to as Heaven's Will.
If one wants to form a contract with a spiritual creature beyond the limitations of their affinity, then they must find some way of overcoming this so-called "heaven." Unfortunately, from the moment a living thing is born on this planet, the laws dictated by the planet's mysterious will seems to be imprinted into their soul.