Brienne represents everything that Cersei wants, but can’t have.
She is a woman who successfully broke gender traditions, evidenced by the sword she carries.
She’s independent and more in control of her own life than even the Queen.
Despite her warrior nature, she’s retained a very innocent (arguably naive) view of the world. Contrast that with Cersei’s jaded outlook and paranoid scheming.
She is the perfect foil to Cersei.