Character Analysis
Billie is the only female character in this movie who can hang with the best of the conmen (and, you know, not attempt to assassinate them). In fact, Billie's actually Henry Gondorff's employer…but she's still happy to help Henry out with some casual pickpocketing now and then.
When we first meet Billie, she's a little standoffish to Johnny. But everything changes once he uses the name Luther Coleman. Luther's name is the magic word, and Billie welcomes Johnny into the fold:
BILLIE: Why didn't you say so? I thought maybe you was a copper or somethin'.
Billie's super loyal, utterly reliable, and has some business savvy when it comes to managing brothels. As Henry says:
HENRY GONDORFF: Great little countess, that Billie. Runs a good house up there, too.
She's also an expert con artist herself, she has the resources to give Henry and his conmen friends a place to plan, and she thinks on her feet. When Lt. Snyder tries to look through the rooms in her brothel she dissuades him by saying,
BILLIE: You'll be bustin' in on the Chief of Police just up the hall.
In short: Billie's more than a match for the conmen she runs with. And that's why we think that Henry and Billie have a real shot at lasting happiness: we imagine them as cynical retirees, fixing games of shuffleboard and conning family-style restaurants into giving them discounts on the early bird special.