Mirrors are more of for people who have strong traces of E.C.O. and the three magic families. They are a reflection of the person's soul, and will be the opposite of the person. Catloneas can merge their soul with their mirror like Parasite did with Trauma to make them stronger. Mirrors if not merged with the person's soul can be a warning to the person that something really good or bad is about to happen. Catt had a mirror that would appear over water when something terrible is about to happen. For the two other magic families they are more used as warnings, or a guide. For people with higher traces or E.C.O. they are used mostly as guides to help the person get to somewhere with something useful or to keep them away from danger.