User blog comment:IamJakuhoRaikoben/Chapter 431 & 432: My sword is... & Brair in Love, Review./@comment-209.129.168.31-20150502004301/@comment-7418318-20150502171449

Why? It's not like Gray is radiating the cold (or rather leeching the warmth from the air) all around him and neither is she using ice magic, so why would she need to get used to the cold?

I think that she got sick from sitting out in the rain is pretty clear evidence that she is in fact NOT used to the cold, because if she trained herself to withstand icy temperatures, sitting in the rain should be easy-peasy to withstand without getting ill.

I think this is just her copying her beloved Gray to feel closer to him.