The cold malignity of her manner sent an unpleasant chill down Tuppence's spine, but she was not going to give in to it.
Her icy aura gave Tuppence all the more confidence that she needed. 
mnli hypothesis: Her icy aura gave Tuppence all the more confidence that she needed.  premise: The cold malignity of her manner sent an unpleasant chill down Tuppence's spine, but she was not going to give in to it.