Bill Collins is an agent with the Chicago office of the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF). His official assignment is to dismantle a major gun-running operation, but his investigation has repeatedly stalled, leaving him with few leads and a great deal of professional frustration. This dead end leads him to take an unorthodox and ethically questionable approach: he blackmails the skilled bounty hunters and gun shop owners Rally Vincent and May Hopkins into working for him. He holds the threat of revoking their firearms license and prosecuting them for explosives violations over their heads to secure their full cooperation, a tactic that immediately establishes an adversarial dynamic.

In terms of personality, Bill Collins presents himself as a sleazy and nosy government agent with a swaggering self-image reminiscent of a prohibition-era lawman like Eliot Ness. His manner of dress and way of speaking are meant to project an image of being a bold, no-nonsense investigator, though this often comes across as hammy or overly theatrical. He attempts to use charm, particularly with Rally, but has little success in this area and is generally inept in the romance department. Despite his abrasive and manipulative exterior, his underlying skill is in investigation. He is persistent, determined to solve his case, and willing to get his hands dirty by operating outside standard procedures when necessary.

Collins's primary motivation is to successfully break the gun-running ring. However, as the case deepens, his motivation becomes more personal and professional when he uncovers a conspiracy within his own organization. After a key witness and several ATF agents are murdered in a safehouse by an assassin using his own keycard and access codes, Collins realizes that the corruption extends to his superior, George Black, and connects to mayoral candidate Edward Haints. This discovery transforms his role from a desperate agent using coercion to a man fighting to expose the truth within a system that has been compromised from the inside.

The central relationship in his role within the story is with Rally and May. Their partnership is built on mutual necessity rather than trust; Collins needs their unique skills, and they have no choice but to comply with his demands. While he tries to direct the investigation, Rally frequently insists on operating without his interference, creating a tense dynamic. However, this relationship evolves as the conspiracy unfolds. Despite the rocky start, Collins provides crucial information to the pair, passing along a watch and paper that contain a backdoor to a website with incriminating arms transaction ledgers. For his part, he proves capable in the field, saving Rally's life on multiple occasions, even if she would never admit to needing the help.

Throughout the course of the investigation, Collins demonstrates notable development. He begins as an agent in over his head who resorts to blackmail for results. As the bodies pile up and he realizes his own boss is complicit in the crime, he becomes a more determined and self-reliant investigator. He does not simply sit back and let the bounty hunters do all the work; he insists on investigating the matter himself, even when it gets him into trouble. His notable abilities include a sharp investigative mind and a knack for handling high-pressure situations, proving himself capable of keeping his composure and acting effectively when lives are on the line.