Is this the instrumental case? And if so, this is a feminine couple/pair?
Yes, it's Instrumental. The word "para" (couple) is feminine no matter who is a part of this couple.
Pair isn't working
I still can't believe that English can use "pair" about people, but apparently it can. OK, added.
Usually it's with the indefinite article, "a pair" e.g. they are a pair. = they are a couple.