Remus could tell she was nervous despite her tone. Actually, there seemed to be an air of tension all around and when he relaxed his attention on her, he could see that Louis was her partner. He was shocked as he tried to process this, wondering what a Weasley was doing with Bellatrix, especially when he admitted to bringing her here.
He sniffed a little contemptuously at her words, but backed off. Moving around them and retreating to where Sirius stood, he remained alert and protective beside him, now watching the young Weasley. He may know and like Louis and he had no immediate grudge against Bellatrix, but at this moment it was obvious to him there were sides to this conflict, and he would defend Sirius the only way that made sense to him.