Bill Gains is a junky. He meets Bill Lee after they are both addicted to junk. Gains was raised in an upper-middle class home—his father was a bank president. Gains makes a living by stealing and pawning overcoats. He is described as a composite of negatives and, thus, nearly completely undetectable to the average citizen, even when he's wearing their overcoats. Physicallly, Gains is described as having lifeless, pale blue eyes and a malicious, childlike smile. Unlike most junkies, Gains enjoys seeing other people develop the habit, and he hands out drugs on credit to younger users to get them hooked.
Bill likens Gains to the parish priest of heroin. In the novel's conclusion, Gains leaves the United States for the safety of Mexico. He uses drugs on the plane by drilling a hole into his leg and pumping morphine through the wound—Bill views the act as an astonishingly proficient procedure capable of being performed only by such a legendary figure as Gains.