Every single cop movie has this trope: a detective walks into his chief’s office with a civilian, the chief goes into a rage-fueled rant, pauses and asks “Who the hell is that?” (or “Who the f*ck is that?” depending on the movie rating and if Mark Wahlberg is the chief) to which the cop responds, “It’s cool, Chief. He’s my C.I.”
The reason the C.I. (or confidential informant in official movie-cop talk) is with the policeman is because he is a flight risk as the informant is sure he will be killed if he testifies against his criminal boss. How did he become a C.I.? Well, I’m not about to tell you; you’ll just have to come along for the ride. This is the insane career path of a confidential informant.
See Also: 4 Jobs for the Criminally Insane