Translated as other self or an alternative personality it always refers to two parts almost with the same type of personality in psychology or of an imaginary friend of someone, but that is a psychiatric issue. The most common sense is the other self of you as simple as it sounds. Cicero described it as a second self or a trusted friend. Fiction refers to good or evil existing in one person. There also can be an explanation in the existence of a subpersonality.
Basically all of us consciously or not have revealed sometimes the alter ego. Some alter egos are better than us but most of them are only the projection of whom we wanted to be at a certain moment in time. You could say that Batman is also an alter ego and in fact this may lead to the idea of a double life. It is not like that. We all have alter egos and we are all under the impression that this is somehow the inner us or the invisible self.
One should ask if using his or her alter ego means being masked or making the real ego to be a spectator. It can be both of these but it depends on each person whom he chooses to let to be in charge. Curiously the alter ego and the real one are friends not enemies but they choose to show different faces of an individual, but very close faces in terms of personality.
Does everyone have an alter ego? I am sure that everyone has its own ego and at least one more and very sure it is not pathology. The alter ego is also a perfect substitute and basically whenever you do not want to be yourself anymore you can use you alternative ego. It means you are doing it consciously, so it is not like you do not know what you were doing. It is like when people say to you: What was on your mind, it seemed you were not you?! It is definitely not you, but the other you.
To check if your alter ego is there put yourself in different situations consciously and get in charge with things you normally do not get. So, test yourself! For example try for a day, or for a week, or more to play a life role. You will see that this is not you but your alter ego. And I am sure you all can do it and pretty sure you all did it times ago. The main idea is to come back to be yourself again after enjoying being the other self. What is the purpose of the other self? That is the question all of us should compulsory answer, even when we might not like our answers.