Detective James "Jimmy" Stills was a corrupt police officer, and a member of Diane Hansen's ring.
Stills was a member of Diane Hansen's ring, and took a leadership role among the men. After his kidnapping attempt of Michael Pope was stopped by John Reese, his badge was taken by the latter. ("Pilot")
Stills was tasked with executing James Wheeler, Hansen's ex-boyfriend and co-Assistant District Attorney who suspected of her shady business. Stills brought along with him an ex-con Wheeler put in jail to frame for the murder. Learning of the presence of Wheeler's son, Henry, Stills was confronted once again by Reese, who warned him not to attack the father and son. After a brief stand-off, with Stills using the ex-con as a human shield, Reese shot Stills and placed his body in the trunk of Detective Fusco's car. ("Pilot")
Under pressure from Reese, Fusco transported the body and buried Stills in a shallow grave at Oyster Bay where his body remained until Internal Affairs received a tip that Fusco had killed him and buried him there. When Internal Affaris attempted to retrieve the body, however, it was missing, apparently removed by Detective Carter with the assistance of Reese's dog, Bear.
Stills relationship with Fusco began when he was Fusco's partner early in Fusco's career. He was responsible for drawing Fusco into his criminal activities and HR. ("In Extremis")