Legendary gangster Jack Diamond is the novel's protagonist and a kind of Irish- American hero like Finn McCool and Jesse James. Finn McCool is a legendary hero of Old Ireland, a huge giant of mythological proportions. Jack, one of the most famous men in America, also has a larger-than-life image that he often says is exaggerated. Jack is compared to Jesse James, the famous outlaw and American legend. Both Jack and Jesse James employ lawless violence to accomplish their ends. Jack grew up in a poor Irish-American family in Philadelphia, and was raised to be a devout Catholic. Much of Jack's own internal struggle with his corrupt lifestyle stems from Catholic guilt. Whenever Jack is harassed by the press and after he starts getting into legal trouble, he persistently claims that his disreputable image is a fabrication of the press and that he's really not such a bad person. Marcus states that his favorite stories about Jack are usually the lies and tall tales. A good portion of this book consists of anecdotes people have told Marcus over the years—stories that may or may not be true. At the beginning of the novel, Tipper, Flossie, and Marcus all disagree about the manner in which Jack was killed. Legs partly weaves an American legend that combines the true story of Jack Diamond's life with rumors and stories about his existence. The novel is narrated forty-five years after the events it describes took place, and this distance emphasizes Jack's position as mythical hero of a bygone era.
Marcus Gorman, an Albany Democratic lawyer and the book's narrator, leads a pretty dull life until he meets Jack. Marcus decides to take on Jack as a client right when he realizes how much his life needs a little excitement. Marcus's attraction to the dangerous aspect of Jack's personality blossoms when Jack lets him shoot a machine gun. Initially uncertain about handling a weapon of mass destruction, Marcus does not want to let go of the gun after he pops off a few rounds. Enchanted by the gun's lethal power, Marcus finds himself living on the edge vicariously, through Jack. Marcus lives a vicarious sexual life, too, ogling the bodies of Alice and Kiki.
Later, when Marcus carries Jack's money on the way to Europe, he starts acting like Jack, seducing and making love to an anonymous woman. By being close to Jack, Marcus feels a rush of the electric energy that he sees even in the way Jack walks around. At the end of the book at The Parody Club, Marcus starts collaborating with Jack and his "gangster stuff" and is overcome with a passion that he ends up satisfying through a sudden, random, and one-time sexual encounter, this time with Flossie. Between Marcus's quiet bragging about his ability to win Jack's trials and the boasts of his Jack-inspired sexual conquests, Marcus glorifies himself almost as much as he glorifies Jack. The sexual nature of the exhilaration Marcus experiences as a result of his association with Jack, and the fact that Marcus also appreciates the sex appeal of Alice and Kiki, suggests that Marcus feels some kind of homoerotic pull towards Jack.
Jack's wife, Alice, is a homemaker. She has furnished their home; she cooks Jack's meals and takes care of his dogs. She surrounds Jack with religion. In general, Alice takes care of her husband. Jack says that Kiki wouldn't be able to take care of a dog, that she would let it die. To Jack, Kiki is the party- girl-sex-object with a small brain and big breasts.
Alice likes the idea of turning Jack into a traveling evangelist-showman. Jack's occupation bothers her, but she likes his notoriety; were he to become a showman, he could carry with him all of the notorious legend that appeals to Alice, but without the murdering and drug smuggling. If he were to preach about having a religious turnaround, Jack could admit to all of the past wrongdoings that make him infamous. The idea particularly appeals to Alice, because it would mean Jack was taking on a holy role, and Alice could finally feel she was married to a devout Catholic.
After Jack dies, Alice creates her own show about how crime doesn't pay, which is essentially a rehashed version of the show Jack could have taken on the road himself. In a sense, Alice is trying to atone for her husband's crimes. Still, Alice never leads a saint's life, even after Jack's death. Marcus suggests that Alice maintained a few of her husband's underworld connections. She also gets union kickbacks, likely because she threatened the unions. Alice met a violent end.
Kiki, Jack's voluptuous showgirl mistress, is short on brains but big on sex appeal. She realizes that she does not have a particularly bright future as a dancer and that hooking up with Jack is a good move. Kiki considers leaving Jack at times, however, bothered by the tales of his no-good doings, his habit of leaving her under guarded supervision, and nearly witnessing his bloody demise. At one point, Kiki does disappear, and it seems as if she could be gone for good. In many ways, Kiki is attracted to the same elements of Jack's notorious image as Marcus is. The fact that Jack is a bad man turns her on. She enjoys licking his scars, and only loves him more when he says he looks forward to dying young. Kiki relishes the secondhand attention people lavish on her in Jack's presence. Being Jack's broad makes Kiki more famous than her dancing ability ever could. Just being seen with Jack is not enough for Kiki; when Jack is shot, she makes sure to capitalize from the accident by publishing her memoirs in the paper. In particular, Kiki likes the way that being Jack's woman makes her a sex symbol.