This notion of the "specular ego" was first developed in the essay, "The Mirror Stage".
The term "counterpart" plays an important part in Lacan's work from the 1930s on, and designates other people in whom the subject perceives a likeness to himself (principally a visual likeness).
The counterpart plays an important part in the intrusion complex and in the mirror stage (which are themselves closely related.
The intrusion complex is one of the three "family complexes" which Lacan discusses in his 1938 article on the family, and arises when the child first realizes that he has siblings, that other subjects like him participate in the family structure.
The emphasis here is on likeness; the child identifies with his siblings on the basis of the recognition of bodily similarity (which depends, of course, on their being a relatively small age difference between the subject and his siblings).
"Imago of the Counterpart"
It is this identification that gives rise to the "imago of the counterpart."
The imago of the counterpart is interchangeable with the image of the subject's own body, the specular image with which the subject identifies in the mirror stage, leading to the formation of the ego.
Formation of the Ego
This interchangeability is evident in such phenomena as transitivism, and illustrates the way that the subject constitutes his objects on the basis of his ego.
The image of another person's body can only be identified with insofar as it is perceived as similar to one's own body, and conversely the counterpart is only recognised as a separate, identifiable ego by projecting one's own ego onto him.
In 1955 Lacan introduces a distinction between the "big Other" and the "little other" -- or the "imaginary other" -- reserving the latter term for the counterpart and/or specular image.
The counterpart is the little other because it is not truly other at all; it is not the radical alterity represented by the Other, but the other insofar as he is similar to the ego.