Halstead is the brother of Dr. William Halstead, a main character on Chicago Med.
In Chicago PD
Jay Halstead had been in the Intelligence Unit for a month when the show premieres. A skilled marksman and an Army veteran, he provides the tactical know-how and is often the designated sniper covering for the team when needed. His interrogation tactics are blunt and tough.
Halstead accompanies Lindsay to her high school reunion. When Lindsay realizes she's out of place, the two instead go to dinner. There, she opens up about her childhood. It is also determined that Lindsay prefers to keep their relationship purely business because her boss Hank Voight (fatherly figure to Lindsay) doesn't tolerate relationships in the Intelligence Unit.
Lonnie Rodiger had killed a young boy who Jay was very close with since it was his ex-girlfriend's younger brother. But Lonnie's dad covered it up and took the blame although everyone knew Lonnie did it because he was a child rapist and pedophile. Jay strongly hated Lonnie and his father so when Lonnie Rodiger is found dead, Halstead is a suspect. Voight pulls Halstead's badge and has Dawson keep an eye on him. It is later revealed that Lonnie's father, Phil had discovered child pornography on Lonnie's computer and confronted him. The resulting argument ended in Lonnie being strangled with a belt.
When Lindsay accepts an offer from the Drug Enforcement Agency to join a new task force, she and Halstead begin a romantic relationship because they are not working together anymore but keep it on the down-low because they are both worried what Voight's reaction to the two of them dating will be. The two end up dating for a few episodes but when Lindsay returns to her job at Intelligence the two are very cautious about being caught. Halstead recommends coming forward and just telling Voight but Lindsay refuses and their relationship ends. Hank eventually figured out they were dating but never told Halstead and Lindsay that he knew.
In the last several episodes of season 2 Lindsay spirals downhill and turns to alcohol and painkillers to deal with her guilt and grief over Nadia's death. Halstead persistently tries to maintain contact with her, even after Voight had given up. She turns in her badge to Voight, who declines to accept her resignation and files a 3-day leave of absence instead.
It turns out the the 3-day absence had actually turned into weeks, in Life is Fluid, and Voight had given up trying to convince her to come back. Jay tracked her down to a club and told her she needed to come back because her 'grace period' was ending the next day, or she needs to tell him that she's no longer his partner. She doesn't do either and leaves him standing.
In the same episode, Jay goes undercover to take down a drug operation with the help of the leader's brother. However, both are made when the leader finds out that his younger brother had been seen with police, figuring out it was Jay. He kills the brother and his men grab Jay and take off. The team desperately chase Jay when they find out something is wrong but Jay is gone by the time they get there. Intelligence is then sent a CD containing a clip of Jay being tortured. Voight is upset to see this and a subsequent phone call sees the men ask for all of CPD's CI files in exchange for Jay's life.
Ruzek immediately accepts the demands when the phone call ends and says it isn't a matter of debate, but Antonio tells him that most of the CI's are his and that he can't have their blood on his hands. Ruzek accuses him of wanting Jay dead instead but Alvin breaks up the fight. The team make fake files but the trade-off has to be by a female officer. Alvin calls Erin and tells her about it and she wants to do it. Erin comes to the station and demands she be the one who goes, but Voight refuses to let her but she's insistent so she ends up going. She exchanges the files but first demands to see Jay, who has not been brought to the trade-off, so Erin goes with them to see him while the team watches. She goes to the house and Jay is brought in bruised and very injured. She stays with him but the fake CI files get caught and the men are about to kill Erin and Jay but they fight back and the leader ends up getting killed by Erin.
Erin returns to Intelligence afterwards. They remain partners. Later in the season, Lindsay and Halstead get back together. Voight, previously against relationships in Intelligence, states that he no longer cares for that policy, mainly concerned with Lindsay's well being, which he entrusts Halstead with.
In season 4, Halstead tells Lindsay "Cases come and go, bosses come and go, jobs come and go, but I don't want you to come and go" and asks her to move in with him. They then decide to move into the apartment Lindsay already owns. Later once Lindsay and Halstead started living together they admit their love for one another.
In the episode "A Shot Heard Around the World" Jay isn't thrilled that his coworker and friends Antonio Dawson is leaving the unit. Dawson tells him that knowing the cop Jay has became made the decision easier to leave the unit.
In Little Bit of Light, Jay moved out of the apartment they shared to go stay with Will. He said that he wasn't proud of the person he was in the past, and that even though Erin may think that he's moved on, he hasn't, and he needs time to sort his problems out, like his marriage to Abby.
They both avoided each other in later episodes until Jay told her they shouldn't have to act like that around each other. They remained on friendly terms later, and Jay joked about a married couple that couldn't live together without fighting and so had to live across each other and how their marriage was perfect because of it, and suggested the idea to Erin, who laughed it off.
In Army of One, Bunny got back in touch with Erin. Jay told her he was suspicious of Bunny, and Erin said she would sort it out herself. Voight saw their disagreement and paired Jay up with Olinsky and Erin with Hayley Upton. However, during a case, Erin lashed out at an offender when she threatened him with a gun.
Jay wasn't happy with the arrangement, and Olinsky told him Voight should've done it long ago. Jay then asked Voigjt how long the arrangement was going to last, and Voight told him not to ask him that again if he wanted to keep his job. Later on, Erin was called to a review board.
In Fork In The Road, Jay asked Will for their mother's wedding ring because he was planning on proposing to Erin. Will told him they weren't even together, and Will said now was the only time he felt it was right because she was in trouble and he should be there for her. Will gave him the ring.
He couldn't propose when he met her, and planned on doing it at Molly's that night. However, Erin had taken the job in New York and didn't go. Jay called her, and she was on her way out - she held onto his Army Tag and rejected the call, implying she wouldn't meet him before she left.
In Reform, Jay has a hard time moving on with the fact that Erin has left. He is told by Voight to let it go and move on, because Erin has. During a case later on, Jay shoots a man and then finds a young girl bleeding out. They take her to Med and Will tells them there is a chance she'll make it. However, investigation then showed that the bullet Jay shot the man with went clean through him and a door and then hit the young girl. Jay is devastated to hear this, and insists he hit a man.
This is proven difficult to prove, and Jay is branded a "racist" as a video of him punching a black man goes viral. The alderman behind it admits he doesn't know the true story, but doesn't care. This is until they finally track down the offender. Voight spends a lot of time trying to convince Jay not to worry and to go home, which he refuses to do. Jay then finds out that the young girl has died, and feels extremely guilty. Voight then takes part in a side deal with the Alderman to make sure Jay's bad image is erased, which no one else knows about. Jay then went to see the young girl's mother later that night with a necklace the girl left behind.
Later on in Season 5, Jay was shown having nightmares and impulsivity issues, which angered his partner, Hailey. However, things worsened when Jay went undercover and began dating the sister, Camila, of the man Intelligence was trying to capture. He kept using his alias until a woman died outside a nightclub that he and Camila were at. He lied and told Voight that he was just undercover trying to get more intel, but hadn't officially put it in the books. Hailey wasn't convinced but went along. This was until plans to arrest Camila came about. Convinced that she was innocent, Jay called her and told her to leave town, even telling her he would go with her. However, Hailey and Voight found him and convinced him to help bring her in. She was partially innocent, but her relationship with Jay would've ruined his career. Hailey convinced her to not say anything about their relationship in exchange for her freedom.
In Chicago Fire
Halstead worked undercover in a sting to take down a ruthless landlord who threatened to burn down Molly's and Game Day. This resulted in his being shot in the shoulder, but a successful take-down.
During his stint undercover, he had a brief romantic relationship with paramedic Gabriela Dawson, sister of Detective Antonio Dawson. However, after he revealed he was moving to Intelligence, Dawson ended the relationship.
In Chicago Med
In 4x09 of Chicago Fire, Herrmann is stabbed by a paranoid worker - Freddie, also an ex-con, that Herrmann had hired after requests from Joe Cruz. Freddie is offended by a joke made by Herrmann and stabs him and runs off.
In Malignant of Chicago Med, firehouse 51 and the team of Med is there to treat Herrmann. Jay's brother, Will, is the attending with Dr. Rhodes. Things go sideways when Rhodes slightly delays in treating Herrmann, leading to a complication. Severide and the rest of 51 push on getting answers, leading them to ask Jay to ask Will whether Rhodes made the right call. Jay asks Will, who tells him he personally thought that Rhodes delayed treating Herrmann. Jay tells Severide, who calls out Rhodes in front of everyone. Rhodes is upset and figures out it was Will and they argue. Will tells Jay their discussion was confidential, to which Jay replied he never explicitly said this. Will tells him he won't make that mistake again, while Jay replies he also won't be passing on information from crime scenes to him anymore.
Jay and Will are on good terms and very close, but it is apparent that Jay still harboured some lingering resentment over the fact that Will was not around when their mother passed away. Will was out "partying" according to Jay, who was deployed. Jay says that unlike Will, he came home.
In The Number of Rats, the SVU crossover included Will being called in for questioning by Voight and Olivia Benson without telling Jay the motive behind it. Olivia believed Will was connected to the brutal murders of young women and saw him as a suspect because he'd been around the areas at the time. Voight, who was also not happy with the suggestion, didn't tell Jay this, who only found out after Will was ambushed by the string of questions when he was brought in to "help". Jay was furious, extremely protective of his brother and demanded Will be taken out. Will tried to say he would help and was ready to answer the questions but Jay didn't let him, and forced him to leave and not say anything.
Towards the finale of Chicago Med and Chicago PD, Jay's relationship with Erin was rocky, and Will was having trouble with Natalie, who he clearly had feelings for. In Divide Us, Will had a small argument with Nina, who was feeling ignored, and Natalie, who was upset after she told Will they were more than friends and he said they were "good friends" then. Will felt like he needed to give more time to Nina, and cancelled going to a game with Jay they'd planned, so he asked Natalie to go with him. She rejected him at first but agreed to go. Later on, in Love Hurts, Natalie asked Jay to go with her to Noah's party. He asked Will if he was okay with that, and Will looked uncomfortable with it but said it was fine. Jay saw it and said he'd tell Natalie he can't come. He tells Will he needs to sort his life out, referring to Nina and Natalie.
Jay wass estranged from their father, Pat Halstead, for reasons which have yet to be revealed and had not spoken to him for almost two years, as he stated in "Get Back to Even", which changed in Generation Gap.
In Generation Gap, Jay phoned Will to come home to where their father was. When Will got there, Jay was by their father's side, laying on the sofa watching TV. He told Jay he didn't need to call Will and he was fine, and Jay said "you missed two days of work. You never miss work!." He lets Will take care of it with Connor and goes back to the station. When Pat wouldn't listen to Connor, Will told him surgery was a long-term solution to the heart problem, but Pat constantly dismissed Will, which deeply upset him. He told Connor that their father saw Will as "somewhat of a disappointment" because he went to med school instead of straight to work. Connor told Will to ask Jay to talk to him, to which he replied that he won't listen to Jay either, because they have "problems of their own". Jay came back during their dad's surgery and comforted Will, who was more worried about the surgery going bad, which it didn't.
Jay developed feelings for Erin after they started working together. After getting to know a little about her background, they both had feelings for each other, but Voight would not let them work together and date at the same time and so they didn't. However, in Season 2, they started dating and assured Voight it wouldn't interfere with their work. This was after Lindsay temporarily accepted another job offer.
In season 2, they continued to date on and off but always remained close. However, Lindsay broke it off after Nadia's death and left Intelligence, spending her nights in clubs and bars in grief. Jay was insistent on bringing her back and calling her everyday but she refused to do so. Voight left her after weeks of waiting, upset at her choice. In Life is Fluid, Jay was kidnapped and tortured in an undercover case gone wrong. Erin came back to save him, after which she came back to the unit and got back together with Jay.
In You Wish, Jay ordered a DNA test to see whether Jimmy, who Bunnie had reassured was Erin's real father, was her real father. She didn't want to do it but he did it behind her back and gave her the test that showed he wasn't. Jimmy lashed out in anger and Erin was extremely hurt, blaming it on Jay for ordering it in the first place.
In Remember the Devil, Will told Jay that his ex, Abby, is in town and tracked them both down. Jay brushes it off until he sees her outside the station waiting for him. He tells Erin to go and meets up with Abby, who tells him that he needs to sign the divorce papers. Jay was married but he tells her he did sign them, which she says she threw away without filing. He agrees to re-sign. Erin eventually asks him if he's cheating on her. He tells her about Abby and their small marriage that he says was only a joke and it was over quickly. She is upset he didn't tell her about it but he feels guilty for not telling her about it. He meets up with Abby, who says she still loves him. Feeling conflicted, he immediately tells Abby no but goes home and packs. He tells Erin he'll be staying with Will for some time until things are okay. She tells him not to but he says he needs to sort his life out and leaves.
In Little Bit of Light, Jay and Erin start ignoring each other, something that becomes very clear during discussions. Voight pulls Jay aside and tells him that he warned them about this and that if they can't work things out, he's gone. They are still dating. However, they continue to work closely together and do well in the cases.
For the remained of Season 4, their relationship was unclear. They were friends again, and Jay mentioned to Erin that he knew a couple who lived in separate houses and were very happy, implying he wanted to get back together, but he backed off when she asked him straight up why he was telling her that.
Later on, Voight decided to break up Jay and Erin's partnership. He partnered Jay with Alvin and Erin with the new Hailey Upton. Jay wasn't happy with it, but Alvin later told him that Voight should've made this decision a long time ago. He said that working with Erin clearly didn't work, but Jay said he disagreed with that. Alvin told him that it was best for him and his career.
Jay then decided to propose to Erin towards the end of season 4, and Will gave him their mother's ring. However, before having time to even talk, Erin took a job she was forced into taking and had to move to New York.
In Reform, he had trouble moving on, because she left so abruptly. It was then revealed that she did not reach out to him either, because he asked Voight if he'd heard anything from her. Voight told him it was the best and that he needed to move on and consoled him.
Mouse (Greg Gerwitz), was Jay's best friend and part of his team when he was deployed to Afghanistan. They are very close and know a lot about each other. Jay offered Mouse the IT job in season 3. Voight and Platt were hesitant in giving the job because of his jitteriness. However, Platt interviewed him and after he is clearly uncomfortable about talking about his past in the Army Rangers, she gives him the job.
Jay and Mouse are very close, and Will said that he wasn't there for Jay when he came home, and that it was Mouse who helped him readjust to life after war.
In season 4, Mouse starts thinking about going back to the Army Rangers. In "A War Zone", he tells Jay that he's been offered a place to come back and wants to accept. He asks Voight for a favour to clean his record or he won't be allowed back. Voight says he'll consider it. Jay and Mouse have a number of arguments about Mouse going back because Jay doesn't want him to. Mouse says he wants action so Jay tells him to become a police officer like him, but he says he wants to be a soldier. Jay refuses to let him and this angers Mouse. He then shouts back at Voight after ignoring him in front of the whole team. Mouse says this is the only thing he's ever asked for.
Erin then tells Jay that he needs to consider whether he thinks going back will be bad for Mouse or for himself. Jay then tells Mouse at the end of A War Zone that if he wants to, he should.
Mouse is not seen again after this, or mentioned. A lot is still not known about Mouse and Jay's past. For example, there is a particular incident that Mouse mentioned twice, but Jay said he doesn't like talking about it. What happened is not revealed as Jay doesn't like talking about his past.
Hailey was made Jay's partner after Voight split him and Erin up. Their relationship was rocky from the beginning, especially with Jay spiralling soon after Erin left with the breakup and possibly his past. Hailey tried her best to keep him out of trouble by talking to him as well as checking on him with Voight. When he and Camila, a woman he met while undercover briefly, started dating, she was cautious and unsure about him continuing the sting. Later on, when Camila was arrested, she threatened Camila and released her in exchange for her never telling anyone that Jay was dating her, as it would've ruined his career.
- (To Erin) : "Cases come and go, bosses come and go, jobs come and go, but I don't want you to come and go."
- (To Erin): "Who I was back then - I'm not proud of it, and I know that you think that everything is behind, but it's not."