A part corresponding to another part; anything which answers, or corresponds, to another; a copy; a duplicate; a facsimile.
One of two corresponding copies of an instrument; a duplicate.
A person who closely resembles another.
A thing may be applied to another thing so as to fit perfectly, as a seal to its impression; hence, a thing which is adapted to another thing, or which supplements it; that which serves to complete or complement anything; hence, a person or thing having qualities lacking in another; an opposite.
I spend a lot of time working by myself developing songs, but I really need some other counterpart to help me pull it all together, because you go nuts working if I had to finish an entire project all within my own head. Andrew Bird