Kelly Severide and Matthew Casey are coworkers and lieutenants at the fire station. At the beginning of season one, after the death of a close friend, they had some trouble getting along and often butted heads but would come together for the sake of the house. By season 2, they have stopped blaming each other and get along very well. They are often seen putting themselves in danger to protect each other, such as when Kelly went into an unstable burning building to help Casey rescue his fiancée, Hallie, who unnfortunately died. After Casey was injured in episode 2x10, Kelly was the first to notice that he was not healed. He was very concerned about Casey, asking him numerous times If he was okay and offering help. The two are close friends.
Following Shay's death, Severide moves in with Casey and Dawson. Severide and Casey become roommates after Dawson moves out. Some of the strain on Casey and Dawson's relationship comes from Casey and Severide's relationship; with Casey putting Severide in front of Dawson, Such as when Dawson tells Casey to ask Severide to move out and Casey responds by saying Severide "is going to need [him] on his side, so good or bad that's where [he'll] be".
In season 3 episode 15, Severide instantly turns down an order from his Interim Chief Pat Pridgen to spy on Casey. He also tells the chief that Casey is "one of the finest firefighters [he's] ever had the opportunity to work with in the entire CFD."
Severide is the only person on the show, besides Dawson, to call Casey by his first name (even though Mills does call Casey by his first name once). Casey is the only person on the show to call Severide by the nickname, Sev.
Kelly and Leslie Shay are co-workers, best friends, and roommates. In Season 1, Shay was supplying Severide, as she is a paramedic, with pain medication to help with his neck injury. In 1x11, Shay and Dawson were restocking the ambulance at the scene of an MVC when a flatbed tow truck blew a tire and struck the ambulance. Dawson only had minor injuries, but Shay sustained a major head injury. Luckily, she survived, but Severide blamed himself for the accident because they were restocking painkillers Shay had given to Severide. He felt better once she came back to work and the accident was never mentioned again. She also wants him to donate sperm for her to conceive a child. It is suspected that there is a slight attraction towards these two. They have great chemistry, and also have the ship name 'Shaveride'.
In Season 2, when Kelly is interviewed by Voight about the disappearance of Keeler, Voight asks him if Shay is his girlfriend. He responds "She would be if she wasn't lesbian."
At the beginning of Season 3, after Shay dies from a fatal head injury in the building explosion in the Season 2 cliffhanger, Kelly is seen taking her death really hard. He takes a leave of absence from the department for over a month, and refused to come back to work until Casey had gone to convince him to come back. Even when he does return to work, Shay's memories haunt him frequently. Shortly after Sylvie Brett began to work in 51, Severide had a double take due to her resemblance of Shay. An escalating tension from this, to the point he tries hang out with Sylvie to get past Shay due to the fact they look similar. He's often shown over-drinking in bars and sometimes watching home videos of him and Shay doing different kinds of things. In 3x19, when Kelly is seriously injured during the Chicago Med incident, he's shown to be having dreams about her laughing while he's in surgery.
Katie Nolan is Kelly's Sister. He first saw her with his father and he thinks she is his new girlfrind, but Kelly's father tells him that she is his daughter. He meets her and tells Katie that he is her older brother. At first, when the two talk Katie thinks Kelly is like their father but she eventually discovers that he is not and begins to trust Kelly. When Katie graduated Cooking School, Kelly was the only member of her family to attended the ceremony. He also organized a party for her at the fire house. The two become close after Katie was attacked in Tonights The Night, but she moves away and accepts a job at a new restaurant.
Brittany Bake-Sveride was Kelly's wife, the two first met at a craps table when Kelly goes on a weekend trip to Las Vegas. Kelly asks for her help with betting due to her beginners luck. After their second roll of winning, he kisses her. The next Monday, Kelly shows up at Firehouse 51 wearing a wedding ring and seems ecstatic, telling all of his colleges that he married Brittany 24 hours after they met at a Silver Balls Wedding Chapel.
The couple get into an argument in 3x08 because Brittany is scared that Severide will get injured on the job.
Kelly and Brittany decide to end their marriage in Arrest in Transit, after Kelly realises that although they love each other, he became Brittany's escape from her parents, after the loss of her sister. They split up at a train station and Brittany heads home to Florida.