Hallie Thomas Edit
Matthew was engaged to Hallie Thomas. After responding to a motor vehicle accident and delivering a baby there, Casey asks Hallie if she wants children, her answer is no and due to their different choices, Hallie cancels the wedding and moves away.
Hallie returns a few months later and realises that she misses Casey. Hallie and Casey got back together and were getting along well, when she was killed and then left to burn in a fire at the clinic she worked at. He was really sad Casey had a hard time dealing with Hallie's death but he eventually managed to get closure and move on.
Gabriela Dawson Casey Edit
Casey and Dawson were first good friends from fire house 51. After Casey loses his late fiance, Hallie, he leans on Dawson for support - they soon become very close.
Dawson asks Casey to be her date for a Christmas function and at the end of the night they lean in to kiss, however, instead Casey kisses Dawson on the cheek.
Sometime later Casey and Dawson's feelings for each other grow. One night Casey goes to Dawson's house and they get together.
In Season 2 Casey proposes to Dawson.
Unfortunately, Casey and Dawson take a break - caused by their relationship going from boss to employee and fiance to fiance.
During the break, Casey and Dawson become close again and resolve their issues, leading to a one night stand. Resulting in Dawson getting pregnant.
Casey and Dawson then get back together but sadly lose the baby. Dawson and Casey are in love and better than ever even through the tragedy.
Dawson and Casey are now married.
Kelly Severide Edit
Kelly Severide and Matthew Casey are co-workers and lieutenants at Fire House 51. 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 find his fiancé. 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 chief to spy on Casey. He also tells the chief that Casey is "one of the finest fire fighters [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. Casey is the only person on the show to call Severide by the nickname, Sev.