Spirits tend to have free will where that is concerned..... if one takes on the responsibility of guirdianship it is because of what ever reasons that only they may know..... you could of course always ask them..... though it would be rare to get a reply that you could understand.... now as Fero pointed out one can ask a spirit to guard over another..... though as to whether or not they do it is all according to that spirits motivations..... I have heard that spirits can be manipulated through the strength of Will alone... I do doubt this except for particular circumstances.... just because they do not have bodies does not mean that they are powerless.... they also have unique Wills.... this is all that they really have left.... it could also be stipulated that spirits gain strength the longer they adhere to the living world.... gaining in potency and power to rise above such clas distinctions that I have given them..... yet an angry spirit will ALWAYS display its most aggressive power...... in the case of a Type A it might craft an illusion to scare the living shit out of you..... a Type B might begin to move your entire living room around.... possibly even shaking your house to its foundations.... a Type C might cause many large wounds to open all at once on a poor unsuspecting individual..... or even in some cases suck the life right out of you until nothing is left....