Mary SIOUX

she has potential to kill tanya, if you watch the last minute from the movie, she rescued by the leader. She maybe will be more stronger and strong enough to kill Tanya. 

She was getting more and more selfish to the point where she was annoying

I can sympathize with her plight but... the way she goes about everything is just so selfish and unheroic.  I think what bothers me is how she is so contrary to what she espouses. I am not certain it was intentionally written that way and that makes it worse (an accidentally hypocritical character).

Now that I think about it, she's basically a more insane female Eren.

Example of someone who let her emotion get the best of her and later gets corrupted by X.