User blog comment:B214/Volume 22 spoilers/@comment-30319856-20161025113305/@comment-10330822-20161031031447

It was never stated "why" she dislikes Ddraig, in fact, that dislike might be why she is focusing on Issei. We also have no idea what happened to Issei during that timeskip. Also, I feel the spectators are just assuming she is a devil, probably because she is using stuff like devil magic. We have no idea what special powers Tiamat has, plus she can shapeshift into a human-like form like quite a few dragons.

The thing is, if its a new character, or the Innovate Clear holder, I see no point in the dragon mask. The mask only really makes sense if its someone who has reason to hide their identity, like Grayfia or Tiamat. Remember, Tiamat is also a candidate for a familiar, so she may have made a contract with Issei as part of this tournament.