Detective Sheldon Jin worked for the Chicago Police Department Intelligence Unit as a Tech and Surveillance expert. He was murdered at the end of Season 1 by Voight's Internal Affairs handler, Edwin Stillwell.
Sheldon was the computer expert of the team. He did all the computer and IT work, like installing trackers in phones or hiding wiretaps in clothes. While the other agents chase after criminals in an active way, he tracks them down on the computer and informs them about their current location.
Sometimes he does illegal work for Voight and searches for people or interrogate people from their team. Later on it is revealed that he works for Edwin Stillwell by spying on Voight and searching on his computer for illegal material and incriminating evidence to send him up.
He does not do that voluntarily, but is forced by Stillwell, because he supports Jin's sick father with money.