Partners are major characters, and the main allies of Cole Phelps as he investigates the crimes of the city of Los Angeles. As Phelps moves up through the ranks of the LAPD, he will be able to investigate cases from different departments, and will have a different partner for each one of them, specialized in the concerned department.
Partners are capable of opening "inactive" doors through tricking their AI into walking in a certain direction (e.g., if Phelps is on the other side of the glass doors at Harlan Fontaine's Clinic as a partner, the latter may "open" one of the "inactive" doors (the ones that Ira Hogeboom charges through to get to and kill Fontaine).
When you need to drive to a location, you can ask your partner to drive, which will transport you to the location instantly. If you decide to drive yourself, your partner will provide you with directions to the location, but only if you prompt them upon each turn with the X button (Xbox 360), or the square button (PS3), though their responses may come a little too late for a turn.
When you're stuck during a case, you can speak to your partner, who will offer a suggestion to what the next step should be.
All partners have their own cars. However, if you get into another vehicle, they will get out of the default vehicle and join you. If you get into another vehicle to pursue a criminal and leave before they enter the vehicle, they will follow in their own car.
When visiting crime scenes, partners will help Phelps look for clues and evidence. When a partner finds something, he will call Phelps over to take a look.
Team Bondi removed two desks from the final game, which means there would have been two more partners with whom Cole would have worked. The two removed desks were Burglary and Fraud, and while speculation was that they would be released as DLC, the developers came out and said that they had been cut in the early stages of development and had no plans for releasing them.
Due to their friendly nature towards each other in the Vice case Manifest Destiny, Harold Caldwell may have been Phelps' partner in the Burglary department. Roy confirms this possibility if you ask him for a hint before talking to Caldwell in the Hollywood Station, saying that they used to work together.
If you leave your partner at a location, a patrol car will pick him up and drive him to your current whereabouts. Once catching up to you, he will rant at you for abandoning him.
Partners appear to be immune to damage in most situations, as they will remain standing after taking damage that would have been deadly to the player.
While chasing a murder suspect, your partner will attempt to shoot out his/her tires. However, partners will only shoot out a person's car if he/she has murdered or attempted to murder you or someone, and only if on the left side of the fleeing vehicle.
If you commandeer a service vehicle, like an ambulance or trash truck, all partners (except Bekowsky) will complain about your choice of transport.
Sometimes, if/when your partner loses their hat, they will not pick it up, and will only have it again in the next cutscene or end of the case. Sometimes, your partner will only have his hat back in the next case.
If you hurt your partner, their response will be intense, and if they are killed, the case will fail.
You can call your partner to your current location when they are near, but if they are doing something they will tell you to mind your business. If not, they will tell you that they are coming. Here are a few for the characters: