Basically, all servitors have an alignment, and the closer your alignment is to their alignment, the better you will get along with them. This means they will be aggressive towards you less often, and will perform their various skills more often for you. Archons heal more, Devils bash more. You get the idea. How close your alignment is to your servitor makes a good amount of difference. It also seems to affect the amount of mana you have to put into conjurations to get upper level servitors.