Mary has relatively little power in her world. She is lower class, a servant, and bossed around by John Proctor. However, Mary gets a little taste of power when she becomes involved with Abigail and the girls. She feels emboldened by her, and their, ability to effortlessly take down people who thought they held power over her. When she is elected to the council, the position goes to her head.
Miller further complicates her character by having Mary be the only one who can testify in Elizabeth's defense. When John threatens to "whip the Devil" out of her, she pulls her trump card, telling her former boss that she alone had saved his wife Elizabeth's life that day. Having been employed in the Proctor home, she could testify that she had not seen any evidence of witchcraft.
Mary is really a little girl trapped in a woman's life. Her immaturity is evident when she informs John with a telling "stamp of her foot" that "I will not be ordered to bed no more, Mr. Proctor! I am eighteen and a woman, however single!"