Riser's Pawn. As a show of power, Riser had her hurt Issei for insulting him and added insult to injury by commenting that despite Mira being his weakest piece, she was able to easily beat Issei.

Spoiler

She later fights Issei again at their masters' Rating Game, where she became one of the first victims of Issei's Dress Break before being completely taken out of the game by Akeno.

(Source: Wikipedia)