Psyche represents a Sin Eater's power, which relates to bond one has with his Geist. This bond is in the sense of combined will, mortal determination and deathly purpose. A Sin Eater with a high Psyche level is more connected with the afterlife and less with the living world, while a low Psyche seem almost normal to the people around them. This is different from a Bound's Synergy, which is the balance of the bond. This connection as it gets stronger will allow greater amounts of plasm use.