User blog comment:Omega natsu2/Chapter 357: The Nine Demon Gates, Predictions./@comment-112.205.13.191-20131020023036

I predict that Tempesta is Yukino Aguria older sister, Sorano Aguria. She had been captured by Zeref since Yukino was still a kid. I think that her sister (as the other users here commented about human form and demon form) she is also transformed to demon or maybe half human / half demon..