Detective David Mills arrives in an unnamed, decaying city infested with crime after he willingly asked to transfer departments. Mills is immediately partnered with veteran detective, Somerset. He reveals that he has worked on homicide cases for five years prior to the film's beginning.Mills has been described as an idealist. And when asked why he demanded a transfer, Mills responded saying he wanted to do some good.As can be seen during scenes of his personal life, Mills is good-hearted and shows much more optimism than Somerset. When Mills invites his partner over for dinner, it invites the audience into his personal life. We learn he is happily married and enjoys playing with his pet dog, despite inconveniences such as a passing train that aggressively shakes his apartment. Mills and his wife, Tracy, joke about the inconvenience.Mills displays a lot of contrasting characteristics with Somerset, possessing a well-rounded amount of street smart knowledge, as opposed to Somerset, who has a book smart mindset with a reliance on literature and documentations. Mills cares about making change, separating his mindset with that of the entire police department. Somerset once cared but has given up, and Mills unconscious plays the role of helping Somerset regain hope.The compassion Mills exhibits translates into overconfident or cocky behavior to the rest of the cops. Mills is a character controlled by emotion and throughout the film will show short-tempered responses. This inevitably leads to some of his various setbacks in the film.