This kind of mental illness is you are acting in more than one person’s shoe.
Let say you do not have a brother, but deep in your mind you really want to have a brother. For some reason, the tension from having a brother is strong enough to form yourself acting like a brother of yourself. As a result, your personalities are splitting into 2 persons, you and your brother. You will lose control of yourself and act like your brother whenever you in need of your brother.
If you want to be some kind of person like that, then, starting from this moment on, pretend yourself being someone else. It is easier to pretend yourself as a friend of you, since you have an actual figure to copy. If you can iron that image into your mind, and if the tension of being your friend is strong enough, then you will become a splitting personality of yourself and of your friend someday.