Wait so im so confused whats her power and how did she steal his bullshit move idk someone...

Marcia's using an exception magic. She's basically setting conditions which once met earned her the prize she set. In that scene, she tells a sob story wherein when heard by someone, they will feel pity and won't have the urge to kill her. So when they started fighting, Shirone kept on having second thoughts on actually ending her as he have heard the story back then at the pub making all of her conditions later on being met, gaining her the ability to steal Shirone's skill.

Thanks big bro or sis