Detective David Mills is partnered with Detective William Sommerset in the film Se7en. The character is played by Brad Pitt. At the start of the film, he has just transferred to the city police department after many years of working "upstate", a presumably more suburban or rural area. Though he is an experienced and capable police officer, he does not have the level of professional maturity that Sommerset has.
David Mills wants to make a difference in the world. He fought to be assigned to a busier area, so that he could work big cases. He is dedicated and highly motivated, though a little over his head in the case depicted in Se7en. Admittedly, the film shows a once in a lifetime case, that would try any investigator. He is by nature hotheaded and impatient. He is prone to losing his temper and acting out violently when frustrated. As Sommerset puts it, he feeds off his emotions. Though this does not always serve him well, it also makes him physically courageous and a very dogged adversary.
David Mills is younger and more athletic than his counterpart, and as a result he is more prone to physical action. He states that he has been involved in at least one shootout while on the job, and mentions other accomplishments. His physical stats are impressive for an essentially average guy. He engages in a long foot chase and running gun battle with John Doe in the second act of the film, and shows considerable speed and ability. [I added to his SPD stat and gave him a couple of three point levels with small arms, as he made some impressive dodges.]
This write up is meant to depict David Mills before the conclusion of the film. As in the film, he makes an excellent foil for Sommerset. It would be difficult to play him after the conclusion of the film, as I would imagine that he is in prison or some type of an insane asylum. As Sommerset's partner, he could be used in the same situations he could. Both characters are written as competent normals, give or take. Se7en's world has a strong sense of cinematic realism to it. No one, with the possible exception of John Doe, is depicted as being superhuman.