Eddie Mars is a racketeer, a gambler, a "bad guy," and, most importantly, Marlowe's foil. Mars personifies everything Marlowe stands against: he is dirty and crooked, and he is directly or indirectly behind almost every murder in the novel. He is perhaps best described through a passage from the novel itself: "You think he's just a gambler. I think he's a pornographer, a blackmailer, a hot car broker, a killer by remote control, and a suborner of crooked cops. He's whatever looks good to him he never killed anybody, he just hires it done." This is Marlowe's description of Mars to Mona Mars, Mars's wife. Whereas Marlowe does not want to kill anybody and does not often carry a gun, Mars has no qualms about murder—but he always asks someone else to do the actual killing in order to keep the blood off of his own hands.
It is significant that Mars is named after the Roman god of war. His name, then, like Marlowe's, also carries a certain amount of symbolism. Drawing a parallel between The Big Sleep and medieval fairytales of knights, Marlowe stands as the knight and Mars stands as the dragon or evildoer. Going further with this analogy, it might be argued that if Mars is not Marlowe's double or foil, perhaps Mars is Sternwood's double. In this sense, Marlowe is Sternwood's knight, while Canino, Mars's gunman, is Mars's own perverse version of a knight, the degraded knight or fallen angel.