John Sullivan, or Sully as he's often called, is dedicated to his work as a police officer on the NYPD. He tries to teach Davis, his partner, that they are not out there just to arrest people. They must try to solve the problems. Sully believes that you should try to solve the problem before you arrest anyone. He sometimes acts like the town sherif as he tries to keep every thing in order.
Sully married Tatiana, his girlfriend from the Ukraine. Their wedding date was originally set for Sept 11th, so the wedding never happened that day. They ended up having a small private wedding where Davis was one of the witnesses.
Tatiana's son is causing lots of trouble and getting invloved with these gangs. These are the same gangs the NYPD are trying to stop. Sully ended up dragging Davis to this restaurant to get Chevchnko, a guy who is the head of a Russian mob. Things ended up getting out of hand and Davis got shot. Tatiana ended up leaving Sullivan after he yelled at her son and blames him for all the trouble including the reason Davis was shot. We will have to wait and see if Tatiana comes back.