In its basic sense it would be like a little specialisation within the AP class. Basically the AP can take on a Demonic aspect. This demonic aspect depends on the demon in question and the skills/spell/abilities you recieve for it are different and varied.
Dodge could be an aspect of the Harmentia as well as other bits and pieces.
Hide of the demon, cant wear armor but get a dam reduction.
Horns of the demon, cant wear helmet but gain blackjack protection and a damage bonus.
Tail of the demon, cant wear leggings unless felar and you gain better balance so lagging moves like bash effect you less.
Demonic bile, spat at your victim to create confusion, do some damage etc etc

These are throw-in ideas but I am sure that Nepenthe or others could take it and do something nice with it. It would be a way of rewarding RP within the AP class and could be given out similarly like virtues are with the RP suited to the demon that grants an aspect.

Just an idea.

-----Abernyte