How to Say "counterpart" in Spanish
The Spanish word for “counterpart” is “semejante” — C1 level.
English → SpanishC1
nounC1formal
a person or thing that corresponds to another

Examples
Debemos amar a nuestro prójimo y a nuestro semejante.
We must love our neighbor and our fellow human being.
La justicia debe ser igual para todos los semejantes.
Justice must be equal for all fellow humans.
Nunca encontré un semejante que compartiera mi pasión.
I never found a counterpart who shared my passion.
Noun Use
When used as a noun, 'semejante' usually refers to a person, especially in the plural 'los semejantes' (mankind or peers).
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