Tsuyuno Yatsumura

She is a magical girl with the power to stop time. She tells Aya information about the magical girls and allies with her so they can be safer together. Unlike Aya, she does not have a problem using her power to injure or kill people who she dislikes.

In chapter 55 of the manga, she stops time once more to save Aya, resulting in her using up all of her remaining lifespan.
In chapter 103 is revealed that she's Aya twin sister.