counterpart (suomeksi)

kollega (s)
virkaveli (s)
vastine (s: something that resembles something else / one which has corresponding functions or characteristics)
vastike (s)
vastakappale (s: either of two parts that fit together, or complement one another)
vastinkappale (s: either of two parts that fit together, or complement one another)
rinnakkaiskappale (s: law: duplicate of a legal document)