Sadie is Willie's "secretary," lover, and behind-the-scenes power broker. Sadie's big talent is politics; she loves it and is extremely good at it. If she were around today, she would probably be running for some kind of office on her own. She is super-tough, and is constantly battling with the Boss over his other lovers. She is also completely loyal to him, at least until the end. As Sadie admits to Jack, she informed Tiny about Willie's affair with Anne. This revelation led Tiny to call Adam, and caused Adam to shoot Willie.
Sadie presents a stunning visual image with her "chopped off black hair," which is "wild and electric," her "big, deep, powerful black eyes," and her white high heels and tight skirts (1.242). Since she chain smokes, the cloud of smoke that constantly follows her seems almost like a part of her body.
As a character, Sadie is hugely defined by her physical characteristics. Jack makes much of her extremely pockmarked face, and at one point when she is in deep despair, Jack tells us that she looks like a "plaster-of Paris mask of Medusa which some kid has been using as a target for a BB gun" (3.320). We soon learn that her face is pockmarked because she and her brother had small-pox as children. Her brother died, but Sadie lived; her scarred face documents forever her battle. We also learn that her father was a terrible alcoholic who gave neither child adequate medical attention. There is intimation of sexual abuse in her childhood stories as well.
When Sadie reveals this information to Jack, she pushes his fingers into her face, making him aware that she is a Medusa only on the surface. She can't turn men into stone, and her powers are purely political. She's a flesh and blood person, with both a heart and a burning drive to succeed. She feels, quite rightly, that she played a big role in making Willie the star he becomes. By the end of the novel, it seems that perhaps Sadie is on her way to developing an identity not dependent on Willie Stark. She is important to the novel as a character divided by strength and weakness. Like Jack, Sadie is haunted by the shame of the past.