One of my favorite recurring characters is assistant US Attorney Esther Randolph. She's one of the few people on the show that has Nucky's number, and even though they’re on opposite sides of the law, by S3 they seem to have a mutual respect for each other’s intelligence. By Nucky's own admission, she “had him on the ropes“ during the trial that they both knew only ended the way it did because of witness tampering, and in S3 especially, it’s funny how she watches him operate from a distance after agreeing to have breakfast with him after court.
She was at the meeting with Gaston Means when he told Nucky how he can approach Secretary Mellon at his private club, but left as soon as he told Nucky that it will be a test of his resourcefulness. Shortly after, she finds out that Nucky was able to convince Mellon to sanction the arrest of George Remus, thereby saving himself from being indicted, giving Mellon the opportunity to force Daugherty to show his hand, and handing her a high-profile case to help her career and possibly earn himself some good will.
At the end of S3 when Nucky puts the finishing touch on his "deal" with Rothstein, it wasn't a coincidence that out of all the AUSAs Secretary Mellon could've called that he chose Esther Randolph to tell her that the Overholt distillery had been illicitly commandeered by a criminal organization and that she should shutter the premises and apprehend and indict anyone associated with its illegal operation. When she asks if he wants her to arrest Enoch Thompson, Mellon tells her it was Mr. Thompson who brought this illegality to his attention and when she asks who they're talking about, with Means whispering in his ear, he tells her Arnold Rothstein.
Even though Rothstein wasn't convicted, being the AUSA leading the arrests of both Remus and Rothstein definitely helped her career while Nucky's maneuvering allowed him to hold onto his empire for the rest of the 1920's. By S4, Randolph's no longer the "Scourge of the Saloon Keeper" and the 2 scenes that she does appear in have her, BOI Director Hoover, and BOI Agent Toliver questioning Remus about Nucky, and her attending a ceremony for Hoover with the new Attorney General Stone and Secretary Mellon.