Severide graduated at the top of class at the academy and held the record for the youngest firefighter to make squad when he was 23 years old. He and his long-time colleague and close friend Lieutenant Matthew Casey of Truck 81 have known each other since their days at the fire academy but have a falling-out over the death of their friend and colleague Andy Darden, whom Severide has known since childhood, over a botched-up call, which is where the pilot picks up. They work things out by the end of the season.
The son of CFD Captain Benny Severide, Kelly was raised singlehandedly by his mother after Benny left the family when Kelly was in elementary school. Although Kelly has since reconnected with his father, they still share a love-hate relationship. He is often known to the "white shirts" (high-ranking officers) as Benny's son but never used his father's name to advance his career, instead carving out his own reputation as a highly competent firefighter.
His charismatic personality makes him a popular figure with both his colleagues and the ladies. He tends to be casual with his men and can be seen at the squad table or in the lounge chatting or playing cards with the rest of the firehouse. Despite this, he tends to keep his private problems to himself and lets it simmer, which often worries his chief. In contrast to Casey, he is highly adept at reading people, especially his own colleagues, and his observation skills have come to the fore in several arson investigations; in season 2, he correctly predicted that Hadley had been targeting the men of 51 and that Candidate Rebecca Jones, who committed suicide just months after graduating from the academy, lacked the discretion and inhibitions to truly excel as a firefighter (although she was later revealed to suffer from clinical depression).
At the beginning of season 1, Severide is plagued with a shoulder injury that puts his career in jeopardy. He hides this from everyone except Shay, from whom he gets painkillers to deal with his injury. When he finally sees a doctor, he is told that his condition is severe and would no longer be able to keep his job. His girlfriend at the time, Renee Royce, tells him of an experimental procedure, which he undergoes to save his shoulder. After the injury a Paramedic Candidate also threatens his career with a false allegation of sexual harassment, although the charges were dropped after Antonio Dawson runs a background check on her and discovers that she was a fraudster. In Season 2 Kelly discovers he has a half-sister, Katie Nolan, who is a culinary student. He tries to reach out to her but she was initially hesitant as she and her mother had been abandoned by his father Benny. The two become close and Kelly is shown to be very protective of her. In the episode "Tonight's The Night" of season 2, Katie is kidnapped and Severide tries to find her in the following episodes. After she is found, she leaves for Colorado. When Katie's kidnapper disappears, Severide becomes a suspect. However, Benny Severide is revealed to be responsible but is let off by Detective Voight after Benny explained himself.
In Season 3, Severide is plunged into grief with Shay's death during a call (later discovered to be murder and arson). He and Gabby Dawson aid Detective Voight's team in the preliminary investigations, which led them to a series of unsolved arson cases, including the fire that killed Mills' father and seriously injured Chief Boden. The case crosses-over into the Chicago P.D. episode "A Little Devil Complex" and the perpetrator is killed by Antonio Dawson. Severide moves in with Casey and Dawson and avoids the apartment he and Shay shared as he is unable to face the loss of his best friend.
Severide is demoted in season 4 as part of Chief Riddle's plan to make Fire Commissioner by replacing Severide with Captain Dallas Patterson and then ousting Boden as Battalion Chief. The men of Squad 3 remained loyal to him despite Patterson's efforts to win them over. He and Boden are later reinstated as Patterson was revealed to have used his connections to Riddle to implicate the latter for corruption.
A recurring theme for the character is his merry-go-round of relationships and flings with various women. Severide was engaged to Renee Whaley, the sister of fellow firefighter Lieutenant Eric Whaley, but he called it off after she cheated on him with an ex-boyfriend. Lieutenant Whaley initially hated Severide, thinking the latter was the one who broke Renee's heart. Severide had refused to tell him the real story to protect Renee's reputation. After the two get into an intense argument, he finally tells Whaley everything and admitted that he never really moved on from her.
Midway through Season 1, Severide was in a relationship with Renée Royce (Sarah Shahi), a lawyer whom 51 had rescued. She asks him to relocate with her to Spain and he was about to do so, much to the surprise of his friends and co-workers, but ultimately decided to stay in Chicago. In the season finale, she returns to Chicago pregnant. However, Shay hints to him that Renée could not possibly be pregnant with his child given the timeline. Renée confesses to Severide that the baby was not his and they part ways once again.
Severide dated Detective Erin Lindsay, whom he first met as she was assigned to investigate the abduction of his half-sister Katie, and their relationship was becoming serious. With Shay's death in the line of duty (later ruled to be a homicide caused by arson), Severide spirals out of control, isolating himself from his co-workers at 51 and Lindsay. She breaks up with him after he misses their date again, telling him that while she could empathize with him, she could not "be the girl sitting alone in a restaurant waiting for [him]."
In Season 3, while still mourning over Shay's death, he takes a trip to Las Vegas and meets Brittany Baker, a graphic designer, at the craps table. They marry and she moves in with him, shocking his colleagues, as well as Chief Boden. She moves in with him into Casey and Dawson's guestroom. Their relationship and open affection would lead to the deterioration of Casey and Dawson's own relationship. Brittany leaves Chicago as she felt homesick and could not get used to the uncertainty and irregular hours Severide's job as a firefighter entailed.
Severide appears to have a romantic interest in April Sexton (Yaya DaCosta), a main character on Chicago Med and an ER nurse at Chicago Medical Center. They have known each other since they were teens as her parents took Severide in when he was going through a rebellious phase in high school. He formally asks her out on date in the Chicago Med episode "Malignant". However, they later break up as April enters relationship with another man.
Of the ensemble cast of characters, Severide is closest to Lieutenant Matthew Casey of Truck 81 and the late Leslie Shay. He considers Shay his best friend and they shared a very close relationship, having shared an apartment for most of the show.
Severide and Casey have known each other since their academy days and presumably graduated from the same class. In the pilot, their mutual long-time friend and co-worker Andy Darden is killed during a call and causes a rift between the two lieutenants. The episode flashes forward a month later and they are still at loggerheads with one another, with the entire crew at 51 as well as new Candidate Peter Mills caught in the middle. They eventually work it out by the end of the first season. Whenever one of them suffers a loss, they would go out to the driveway to talk over a quick smoke, as both are men of few words, especially regarding their private lives or emotional state. (In Season 3 Episode 10, Severide and Casey go to a bar to get over their break-ups with Brittany and Dawson respectively and attempt to pick up a pair of girls named Chloe and Yuma, a reference to the film 3:10 to Yuma, whose screenplay was written by Fire writers and co-creators Derek Haas and Michael Brandt.)Chicago P.D.: "Chin Check" (January 22, 2014)
Chicago P.D.: "At Least It's Justice" (April 2, 2014)
Chicago P.D.: "8:30 PM" (April 30, 2014)
Chicago Med: "Fallback" (December 1, 2015)
Chicago Med: "Malignant" (January 5, 2016)