Kelly Severide is the Lieutenant in charge of Rescue Squad 3. Severide is known as a "ladies man" as nearly every female he seems to encounter suddenly becomes attracted to him, often leading to them going back to his.
Kelly is dedicated to his position in Rescue Squad and he is proud to be there, and is happy that he did not need the help of his father to get where he is today.
Severide has many ex-girlfriends and flings, however the only relationships that seem to last are the ones that are non-sexual. Like his best friend Leslie Shay and his half-sister Katie Nolan, friendships rather than relationships.
Severide and Casey are still dealing with the death of one of their team members, Andy. There's tension between Truck 81 and Squad 3 regarding the situation - Severide at the time blames Casey for letting Andy go in to the fire while Casey blames Severide for not warning them about the vents which caused the explosion.
During this, Severide is suffering from an injury in his arm. He makes an appointment, where the doctor tells him that they will need to operate and he won't be able to work for a year. He ignores the instructions, the doctor warns him the pain and injury will worsen but he refuses to have surgery. Shay , his best friend and roommate, helps him when he asks for stronger painkillers by taking them from the ambulance. Eventually the pain worsens and he asks for more, she tells him to get surgery and stop his childness. He turns to an old girlfriend instead, who gets him much stronger and illegal medication. He keeps taking them until a drug test is ordered on all the firefighters after a case they work on. He tries to delay the test but is unable to, Shay finds out and helps him. She gets very frustated at him and leaves their apartment. After she's in an accident, he asks her to come back but she refuses as he's still taking the medication. He eventually turns to Dawson for help and books an appointment with the doctor and Shay moves back in. Even though the injury is exaggerated at the beginning, he's released quickly and is back on Squad in no time.
Severide reconnects with his father, Benny Severide who comes to visit Firehouse 51 as he served there with Henry Mills and Chief Boden . Tensions arise between him and Chief regarding the death of Henry Mills. Boden tells Severide to tell his father not to come back, but he does so any way. A fight breaks out between Benny and Peter Mills when Benny tells Peter that his father's and the other firefighter's death was his own fault. He also tells Severide to not trust Mills. This reversely encourages Severide to trust Mills even more and even go as far as to visit Mills at home after his and Benny's fight and move him over to Squad 3. He tells him which courses to go to - Chief Boden supports his decision to make Mills the youngest transfer to Squad, this doesn't sit well with Casey, who feels he isn't ready.
Severide is targeted by an arsonist twice before realising that the arsonist is targetting firefighters. He makes the connection that the arsonist is also a firefighter, most likely Kevin Hadley. When he goes to see Hadley, he hints that he's the arsonist but they have no proof, they eventually catch him and he's arrested.
Severide also meets his sister, Katie Nolan, for the first time. He tries to connect with her but she initially refuses, not wanting to be left hanging again like Benny did to her. However, she quickly changes her mind and comes to visit 51, bringing them dishes occassionally, being a chef. Severide is also the only one who attends her graduation to be a professional chef since her mother doesn't have time. During her visits at firehouse 51, she meets Otis and starts dating him with Severide always watching out for her. However, she's kidnapped by 2 men with a vendetta against Severide. They attack her badly and she tells Severide that even though she loves him, she's accepting a job offer in another state and moves away.
Shay also has a hard time after one of her patients committs suicide right in front of her. In an argument, Dawson blames her so she leaves 51 and Severide watches her spiral into a time of darkness and bad decisions. She starts going out with a girl who later robs their whole apartment, including Otis who has moved in with them. This angers Severide as he warned her about the girl. This combined with the stress from the case drives her into depression and she contemplates suicide. Dawson finds her before she does on the command of Severide who tells her to find Shay. They both apologise and Shay comes back to 51 and is happy again. However, the girl comes back to 51 and apologises to Shay and Severide. She doesn't have a lot of their stuff anymore and Severide gets angry at Shay again for not calling the police. The girl lures Shay back in with a story about trying to help her troubled dad who needed the money. This is until Severide snaps when she back into their apartment and kicks her out. Shay gets past this quickly.
Their happiness is shortlived when at a building fire, Shay and Dawson rush in to help a victim. The building suddenly explodes and Shay suffers from a head injury. Severide desperately tries to revive her.
It's revealed that Shay died on the scene and the attempts to revive her had failed. Severide goes AWOL for over a month until Chief tells Casey it's time to go look for him and bring him back. Casey visits him but Severide tells him he's going to start a business by the coast and won't go back to being a firefighter or back to firehouse 51. Casey is fine with the decision but shows him a book that Chief keeps, listing all the lives they saved. This encourages Severide to return. However, when he sees Sylvie Brett for the first time, he mistakes her for Shay until he sees her face. However, the resemblance between the 2 prompts him to get close to her, using her as an excuse to get past Shay.
Severide isn't himself and goes out drinking every night, everyone is worried about him. This is until he goes to Vegas for a weekend and meets Brittany Baker, a graphic artist from Florida. Severide returns from Vegas, smiling and happy, and informs that house that he got married in Vegas. They think he is kidding at first, but soon realize he really did get married, Severide and Brittany were both recovering from tragic losses (Brittany was driving the car it crashed and her sister died in the accident), and their impetuous marriage and connection proves a soft place for them each to recover. However, Severide eventually realizes that Brittany is still hiding from her pain. He hid from his pain by drinking, and came out of it when he met Brittany. But she is hiding from her pain by being with him and avoiding her parents. They have a tearful and tender goodbye scene, as Brittany returns home to Florida to deal with her grief and her family.
During a call, Severide and Dawson find information on Shay's death at the old building. They start to investigate further and find that it was actually arson. The arsonist is caught after a long run and is killed. He and the rest of 51 have a memorial service for her at Molly's.
Severide's close and old friend Scott Rice is transferred to Squad 3. This raises tensions between him and Mills, who also wants his old spot back as he's waiting for his medical clearance but Rice gets there first. Mills is eventually transferred to Squad 3 again but only stays a shift before deciding to leave to North Caroline. Rice is in Squad 3 until Otis starts to accuse him of being a 'ducker' and avoiding tough calls. He initially claims there was a malfunction with his oxygen mask which Otis and Cruz disprove. This causes great tension between Truck 81 and Squad 3, going as far as to Severide nearly hitting Otis at Molly's while defending Rice. However, the accusations prompt Severide to check the mask and he finds Otis telling the truth. Scott is dismissed from firehouse 51.
Severide finds himself in a tough position after he is demoted from his Lieutenant rank after the accusation of him being a poor leader because of the number of firefighters that have either left or been dismissed. Their reasons are very legit but due to the fact that they worked under Severide, he gets the blame. Chief Boden does everything to get his position back and tells him to go to Manager's meeting where his leadership skills can be improved.
Severide has a tough time during his demotion, constantly being undermined by Patterson, who he has been replaced by. Patterson ordering him does not sit well with him as he has a hard time accepting that he's no longer Lieutenant. He even gets suspended for a shift for threatening to hit Patterson when he tries to talk to him after shift. He and particularly Gabby think Dallas is trying to replace/get them all fired. Patterson doesn't understand this accusation when he is asked several times as he's shown to have a hard time understanding what he's doing wrong as he is only trying to do his job but the fact that he is slightly unfriendly makes them all wary of him. Benny gives Severide a file on him to ultimately blackmail Patterson, Severide decides to hand the file to him. Patterson still doesn't understand why he would give it to him but he helps to calm Riddles down after Boden charges at him with accusations, which Severide is surprised to see.
In S04E09, Severide is re-promoted to Lieutenant after Patterson's promotion to Battalion Chief.
Severide temporarily blames Cruz for Herrmann getting stabbed by Freddy for bringing him to the firehouse in the first place and pressures him in to finding Freddy when he runs away. This is until Chief Boden tells him to back off, which he takes into effect immediately as he goes to look for Cruz when he then tries to find Freddy at their 'gang hideout' and tells him to not risk his life unless he wants to be the one in the hospital.
Kelly also develops a brief attachment to Bianca Holloway at CPD when he tries to tell her that a missing body they find is a young girl who disappeared from his neighbourhood a few years ago. She is initially irritated at his efforts in trying to identify the girl and tells him to back off. However, Severide is right and later on, she asks him for a favour - to look after her son JJ - while she prepares for a court hearing on a dangerous gang she had been detecting for some years. He agrees but doesn't know about this until Antonio tells him about it. She tells Kelly she's worried for her son's safety and wants Kelly to look after him. He tells her she can drop him off anytime.
On the day of the hearing, she drops JJ off with Kelly for a few hours. However, CPD is soon at the firehouse and inform Severide that Detective Holloway was shot outside the court. He is upset to hear this but insists that JJ stay with him until his aunt, who has requested not to tell JJ about this, has flown in from California. The officers agree and tell him to stay put. The firehouse help JJ with a school project, trying to divert his attention from his mother, who he keeps asking to call. They let him film them for his 'Day In The Life of a Firefighter' documentary and he tells them that now, he just has to film his mom. A few hours later, Detective Crowley from CPD is back to tell him that Holloway died during surgery. He still insists that JJ stays with him until Mary (Bianca's sister) is here and doesn't let hte other officer take him. Mary arrives later that night and clearly upset and crying, asks Kelly to help her break the news to JJ. They take him to the briefing room next door and tell him with the whole firehouse watching from the window in the common room. He breaks down and Mary takes him away. Just before he leaves, Severide gives Pouch, their dog who has also grown close to JJ, to JJ as he cries in the car. He tells him he can visit or call anytime.
Most significantly, all of firehouse 51 collectively disagree and are against Jimmy's angry attitude about being transferred to Ambulance. After filing a report against Chief Boden and consequently being transferred back to Squad, Severide and Casey are especially irritated by his actions. This causes them to practically ignore him during their next call and he takes the rash decision to go in despite orders and get severe burns. Chief Boden~ sees him in hospital one last time before he leaves.
Severide next has a disagreement about whether a man started a fire in their apartment to kill his wife or not. Severide says he didn't start the fire because of the way the husband kicked him off to go in the burning building to save his wife. Casey disagrees, as he is the one who held the wife's hand seconds before she died and also found evidence of arson. Severide tries to prove the husband's innocence but is proven wrong in the end, after the husband admits to starting the fire but didn't know that his wife was inside at the time.
Severide later has trouble with his relationships and starts thinking about his life. He is then injured in an accident and doesn't get it checked out until Chief forces him to. He meets Chicago Med's Dr Clarke - a past friend of his, who asks him to take a quick blood sample and enroll on the bone marrow registry. He brushes it off for later and leaves. Clarke comes to see him the next day and says that the reason for his asking was a cancer patient of his, who is dying and needs this and he thinks Severide could be a match because of something he told him earlier. Severide again, brushes him off and finds it strange. However, he goes to see Clarke after Gabby and Casey get married. Clarke shows him the woman and Severide instantly agrees. Clarke tells him he'll feel the pain for months, to which he replies, "good".
- (To Shay): "I didn't touch your yogurt."
- (To Shay): "No offense, I'm not taking advice from you right now."
- (To Shay): "That girl is in serious heat and I'm not Gandhi."
- (To Shay): "I don't trust anyone in this world other than you."
- (To Jones): "Jones, you're at the Squad table."
- (To Devon): "I know you're a thief, I know you're a loser! I know you're bad news for Shay!"
- Severide's Badge Number is stated to be 1751 - Revealed in Season 2, during the arson attacks although in Season 1 his badge number was revealed as 1068.
- Kelly's original vehicle was a vintage Black Camaro, written off after Hadley torched it. He then got a vintage blue Mustang.
- His first two years as candidate, he "never looked sideways at the women he worked with" - According to Boden.
- It is mentioned that he was the youngest person ever to join squad at an age of 23 years