"Oni nie są parą."

Translation: They are not a couple.

[Dedumdadum]

Is this the instrumental case? And if so, this is a feminine couple/pair?

[Jellei]

Yes, it's Instrumental. The word "para" (couple) is feminine no matter who is a part of this couple.

[Dedumdadum]

Thanks!

[BaltoSlavic]

Pair isn't working

[Jellei]

I still can't believe that English can use "pair" about people, but apparently it can. OK, added.

[JHeaven]

Usually it's with the indefinite article, "a pair" e.g. they are a pair. = they are a couple.