At first, I imagined a system with five different statistics for the characters, each one "opposed" to two other stats, represented in the form of a pentagram. It made sense thematically and seemed easy enough to grasp on paper; however, it went against another aspect we really wanted to experiment with: the ability for characters to evolve and heal or succumb to their traumas. This meant the stats had to evolve as the characters did, and if they had to be compared and "mathed" against one another every time, this would have considerably slowed down the pace of the game and risked turning into a chore at some point.
So rather than relying on several stat values, we kept only one: the "voice" - representing the demon influence over the character. But this also reduced the potential for characters' evolution. So what I did instead is I designed different "traumas" and "hopes", specific rule-breaking abilities characters could unlock or be afflicted by as they evolved away from, or toward their demon.