Help please! Which is more appropriate?

"Are" is correct, because you are referring to girls who ARE always complaining - she IS one of them.

I normally would say that she is one who IS (the sentence is about her, not some other girls). doesn't matter what she is one of. But you can definitely say it as "are". it depends on which antecedent you wish for "who". Clearly, the sentence has an ambiguity and thus would benefit from restructuring to make clear which choice you intend to mean. declaration of the verb number will work, but it is a bit awkward. If you do not even know, how the heck are we supposed to know?

'' She is ........ complaining.''
''She are ............ complaining.''

Which sounds right to you?