<P> 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 . </P> <P> 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 . </P> <P> 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). </P> <P> 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 . </P>

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