Detective Jason "Jay" Halstead first appears on Chicago Fire as an undercover cop assigned to shadow and take down a local mobster who is harassing Gabriela Dawson, one of the owners of Molly's bar.
During one of their tours in Afghanistan they were both deployed to the notoriously hostile Korangal Valley[15][16][17][18][19] and were in the lead Humvee of a convoy which was attacked, resulting in Mouse being given a medical discharge.
[18] With no familial support, he had difficulty readjusting to civilian life when he first returned stateside and spent a period of time "doing nothing but drink, smoke, screw, fight".
For example, his apartment is spartan and well-kept and he is very physically fit, highly proficient in close quarters combat[27] and able to maintain his concentration on a subject from behind a sniper rifle scope for long periods of time.
A highly skilled marksman, Halstead is proficient with his standard issue Glock 17 and M4 firearms as well as a sniper rifle[29][26][30][31] and has an extensive knowledge of ballistics and explosives.
Likewise, in season 6, he disobeys Voight's order to stay on desk duty upon hearing that the fire his father died in was an arson and continues to pursue the case by himself.
He goes undercover to infiltrate a kidnapping crew composed of military veterans and strikes up a friendship with Luis Vega, his "target", due to their shared background as former Rangers who had been deployed to Afghanistan.
[18] In "Rabbit Hole", the relationship between Halstead and Camila takes a dramatic turn when he unintentionally gets himself in the middle of a drug operation and a murder of an undercover DEA agent.
In the season 6 episode "Trigger", he was visibly affected by the fact that a fellow veteran would frame a local imam and kill innocent people in a deluded quest to prove that the imam was financially supporting terrorist activity; for the first time, Halstead speaks at length about his time in the military and comments that he has learned to live with his PTSD and move on with life, unlike the perpetrator.
[25] In "Outrage", while working undercover in a drug bust, it goes sideways when the team identifies the suspect as Matthew Garrett, a perp who Halstead thought was in prison.
As he leaves the club, Jay describes the guy as “the devil.” A paperwork SNAFU leads to Garrett getting a huge payoff for wrongful arrest, and now he is using it to fund his drug operation.
In "Brotherhood", upon learning of Antonio's addiction in "Descent", he covers for him at the same time as the OID is investigating the events that transpired, infuriating Voight to the point of almost getting into an altercation with him.
Like Joe Cruz from Chicago Fire, Atwater was raised in a gang-controlled neighborhood and had relatives and friends in prison but avoided a life of crime.
They're able to keep it clean till the end of their shift until a construction worker drops a cinder block on the hood from 20 feet high while they're assisting on Intelligence's case.
In "Snitch", it is known that his little brother, Jordan, was born on October 18, 2002, when he, Burgess, and his friends were celebrating his 15th birthday and Atwater keeps a close relationship with the people in his community.
In "Captive", in the middle of Trudy Platt's birthday karaoke party he receives a phone call in which prompts him to leave Molly's to check up on an anonymous person.
The harassment continues in season 8, with the officers (led by Nolan) planting drugs in Kevin's car, beating him in the middle of the streets, breaking into his house, and refusing to send back-up that leads to Adam being wounded from a shot.
Kevin threatens to quit his job and out Nolan as a racist and crooked cop, knowing full well that will ruin his reputation in the community and jeopardize his chances of a pension and his status with his family.
She was previously a training officer (Antonio Dawson was one of her trainees during his days walking the beat) and had passed the detective's exam twice but had to take a desk job after being shot.
It is also implied that while Platt is financially independent, she has long relied on her family's money as back-up and now has to scale back her lifestyle due to her meager salary.
Platt leaves the house so she can be back in time to meet her husband for end of shift and as she unlocks her car door, she is struck in the head and repeatedly beaten.
In the Chicago Fire episode "A Real Shot in the Arm", Mouch uses his and Platt's savings to buy out Otis' shares of Molly's, making her a co-owner of the bar.
Before joining the force, Olinsky served in the United States Army and was at one time stationed in Vicenza, Italy with the 173rd Airborne Brigade Combat Team.
In the Season 2 finale, "Born into Bad News", Alvin is contacted by a young girl from the juvenile center named Michelle and reveals her status as his second daughter.
In "Breaking Point", he is taken into custody for questioning by Internal Affairs when his DNA comes up on the body of Kevin Bingham, the killer of Hank Voight's son, Justin.
As the investigation into murder goes forward, it was revealed to be a hit by a drug runner for the Cali Cartel that Olinsky was trying to take down while he was in a DEA task force who arrested his brother.
She later accepts a position in the unit to fill in for Kim Burgess and later replaces Erin Lindsay following her departure for the FBI's Counter-Terrorism Bureau in New York.
Upton has a history of dating her colleagues: Garrett Thomas, her sergeant whom she was undercover with before she was promoted to detective, Adam Ruzek, and her partner Jay Halstead.
In season 8 episode 3, "Tender Age", Upton is offered a permanent job in NY by the FBI but declines, telling Halstead that he and the rest of the unit make her a better person and that she wants to stay in Chicago.
She has a panic attack about the shooting after a suspect in the unit's current case, attempts to commit suicide in the interrogation room due to her revealing too much information.