Souls

A person's soul is anything and everything that person will ever be. It is all the possibilities within them, deep down. It is what makes them singular and unique from every other person on the planet. A person's soul is their identity and a hunter's mask is a physical representation of that identity. When students first bring out their mask they are granted more power, but this comes from being able to look deep within their selves to who they truly are. This realization is not something that can be faked. A person can't fake their soul-face. Bringing out a mask also makes that person uncomfortable for a while. To look that deep within your self can be disconcerting. You're also allowing everyone else to see you, no-holds-bar. Your soul is you as far down as anyone can get.

That's why masks rarely (if ever) change throughout a hunter's entire life, because you can't change what is irrevocably you. There are a few noted cases of masks changing, becoming twisted and horrible to look at, such as the event at the 8th Library with the cursed hunters. For such a thing to happen, an event must occur that changes everything within a person to the core.