"Brettsey" refers to the relationship between Sylvie Brett and Matt Casey. They developed a close friendship filled with trust during season 7. During season 8, their interactions seem less platonic, but they consciously avoid starting a romantic relationship in order to maintain their friendship. Their relationship becomes much more real in season 9, its romantic phase starting at the end of the season. They date long distance during Season 10 when Matt moves to Portland to foster the Darden boys, Sylvie then breaks up with Matt at the start of Season 11 due to the distance between them and at the end of Season 11 Matt surprises her by proposing and it is revealed at the start of Season 12 that Sylvie said yes, they are now married and have moved to Portland together as of Port in the Storm
Season 3 - Season 6[]
Casey and Brett are friendly towards each other, but they don't share any one-on-one moments. Their interactions are solely professional or linked to their mutual friend - Dawson. This connection to Dawson is the main reason why Sylvie won't act on her feelings and dismiss any affection she feels towards Matt throughout seasons 7 to 9. They respect each other greatly, especially for their professional accomplishments.
Casey is caught between Brett's romantic relations multiple times. Firstly, Otis approached him looking for advice when he had a crush on Brett immediately after Cruz broke up with her. Secondly, Dawson (unsuccessfully) urged him to recommend some of his friends that Brett could date after her breakup with Antonio. He demanded not to be drawn into Brett's dating life ever again.
Season 7[]
They grew closer as friends after Dawson left for Puerto Rico to help the regional relief aid. Brett helps Casey look for a new apartment, after a fire burns down his place, and she is concerned for him when he suffers some trauma after a scary call, giving him advice and offering to be there if he ever wanted to talk.
After he is injured on a call, Brett tends to a burn on his neck, teasing him about his bandage skills. Casey then compliments her work, jokingly telling her that she should work in EMS. The two then share a long look before Brett quickly leaves and Matt is clearly confused. Casey is now unsure about his feelings towards Brett, and when Boden invites him to a dinner, telling him to bring a date, he nervously tries to ask Brett, but is interrupted before getting the chance and then gets cold feet. When he sees Brett and Kyle, a CFD chaplain, at Molly's sharing a hug, he jumps to conclusions and assumes that they had gotten back together. He tells her that they are good together, before she gets a chance to explain that they're just friends. Brett seems upset by this and is confused further. Kyle then proposes to Brett at the firehouse with everyone looking on, including a stunned Casey. She accepts, although seems reluctant. Casey congratulates her, and she tells him that it was what he said at Molly's that made her accept. He realizes that he had unknowingly pushed Brett back to the chaplain, and he is clearly upset by this.
Sylvie dates a man named Kyle Sheffield, a chaplain with the CFD and a friend of Foster. However the pair break up, Kyle citing that their relationship has led to firefighters not willing to confide in him after Brett asks him to talk to Casey about a bad call he experienced. (Fault in Him)
In Try Like Hell, she has difficulty moving on from Kyle and sympathizes with Casey on the subject of tough break-ups. She then attempts to help him out by setting him up with her spin class instructor, Olivia. After a game night with their friends, Olivia says Casey is great, but that Brett should be the one dating him as the two were "meant for each other". Later, Brett confides this to Foster and Kidd, who seem to agree with Olivia's assessment and leave Brett feeling confused about her feelings towards Casey.
Kyle comes to 51 and proposes to Sylvie in front of everyone after he tells her he's moving to Indiana. Kyle then proposes to Brett at the firehouse with everyone watching, including a stunned Casey. She accepts, although seems reluctant. Casey congratulates her and she tells him that it was what he said at Molly's that made her accept. (Casey supposed they got back together when Sylvie greeted Kyle with a hug, so he expressed his excitement about it and told Sylvie that she and the chaplain are a great match. Sylvie doesn't correct him even though she was only friends with Kyle at that point.) He realizes that he had unknowingly pushed Brett back to the chaplain and he is clearly upset by this.
Season 8[]
At the start of Season 8, Sylvie follows Kyle to Indiana following the boiler explosion in I'm Not Leaving You. Brett has moved back to Fowlerton with Kyle, but she is unsatisfied with the slow-paced life of the firehouse there and misses Chicago and everyone at 51. She and Kyle amicably break up and she returns to Chicago.
She returns to Station 51 in Badlands, where she reunites with everyone and Casey is happy to see her back, saying that 51 isn't the same without her.
The two continue to grow closer, and Casey starts becoming jealous when he sees her with another man, Ryan, and assumes they're dating, to which Brett quickly explains that they're just friends, prompting knowing looks from Foster. When he teases her again about Ryan, she talks about her lack of luck when it comes to romance, but he tells her that she deserves to find exactly what she wants in a relationship, resulting in some heart-eyes on Brett's part.
Dawson briefly returns in Best Friend Magic. The whole house is surprised. Dawson attends a fundraiser (connected to her work in Puerto Rico) and she invites Casey to a charity ball. Casey consults Brett on this matter and asks Brett's advice on what he should do, as he isn't entirely sure it's a good idea. She encourages him to go, but is visibly upset. He goes to the ball and he and Dawson end up spending the night together, after Casey clearly states that he'll be fine even if she leaves the next morning, and he will stay in Chicago. Brett asks Casey how the event went, to which he tells her it was great. Foster teases her more, but she tells her: "Stop. He seems happy...that makes me happy."
In A Chicago Welcome, Casey and Brett tend to an elderly victim who had lost his wife in a house fire, and they search for his beloved cat. Whilst they wait, she asks for his advice as she has received information that her birth mother is seeking contact. Not quite knowing what to do, he tells her "I've been impressed by your ability to see the good in people, always. If anyone can handle this kind of bomb dropped in their lap, it's you."
In Off The Grid, she then receives another letter from her birth mother herself and she asks Casey to throw it away for her, to which he agrees, but keeps it anyway. When she later asks about the letter, he smugly shows her the letter, saying that he knew she'd change her mind. She gets angry, to his hilarious confusion, as he didn't do what she asked and asks him to get rid of it again. He throws the letter in the trash, but Brett later comes to his apartment, saying that she's changed her mind and wants the letter back. He seems confused as she saw him throw it away, but she doesn't buy it and asks him if he went back and got the letter after she'd left. He smiles and says "are you gonna yell at me again if I did?" She asks him to stay with her as she opens it, only to find a name and an address. He offers to drive her to the house, for which she is grateful.
During The Tendency of a Drowning Victim, When Brett introduces her birth mother, Julie, to Casey, she misunderstands their relationship and asks Brett how long they have been dating. Brett quickly explains that she and Casey are just friends, but Julie isn't entirely convinced. Brett spends time with Julie, who is happy at getting to know her and teases her for her obvious feelings for Casey.
When a realtor asks Brett out, she seems unsure. Julie tells her that she's hung up on Casey, but Sylvie reaffirms their platonic relationship, and explains that Casey was married to her best friend, so nothing was going to happen. However, she starts to question why she is reluctant to date anyone else. As she leaves after a shift, the relator calls her, and, as she looks on at Casey, she turns him down, telling him that she's "got some things to sort out," having finally acknowledged her growing feelings for Casey
In I'll Cover You, Julie goes into labor and Brett calls Casey for support when the doctor explains that there are some complications. When Casey arrives at the hospital, the doctor tells Brett that Julie did not make it through labor; however, the baby did. Devastated, Brett goes to see her little sister and cries while she holds her.
Casey, seeing that Brett is heartbroken by Julie's death, continues to check up on her, telling her that how she's feeling isn't silly. When Julie's husband, Scott, starts to have doubts about raising the baby alone, he asks Brett to look after her or he'll put her up for adoption. Brett is shocked and completely unsure what to do. She confides in Casey and breaks down in his arms, grieving for her mother. He tells her: "Whatever you decide, I'm here." She appreciates this, but ends up convincing Scott to keep the baby. At Cruz's wedding, she and Casey share longing looks during the exchange of vows.
Season 9[]
In Rattle Second City, the season premiere, Brett and the fire house welcome new paramedic Gianna Mackey. She takes Mackey on a call, with both finding a man who has overdosed. While tending to him, a gun is held to her head by the man's brother, who threatens to kill them if they don't help his brother. Brett stays calm and talks him down before taking the brother to the ambulance to transport him to the hospital. Afterwards, she learns from CPD Intelligence Officer, Kim Burgess, that the brothers are heavy criminals. She later hears the younger brother died and worries about the elder one taking revenge. She worries about him stalking her, especially when she finds her apartment door open. Not taking any chances, she calls Casey and he decides to sleep over to protect her. She and Casey spend the time joking around by playing games and having a good time. They end up having a game night where Casey does an Australian accent. The next shift, the older brother runs the ambo off of a shallow bridge while the ambo is responding to a call.

The first kiss and almost hookup (9x02)
In the next episode, That Kind of Heat, Brett and Mackey are shown to have survived the crash, but are trapped inside the overturned ambulance. After breaking free, they wonder if the older brother is still out there. They climb out to find Casey and Truck 81 has show up. The firehouse had received word that Ambulance 61 was under attack. Casey and Truck 81 rush to their location and Casey jumps out of the moving truck to run and get down to Brett and Mackey. The older brother's car is overturned and the older brother is trapped inside while his suv starts to catch fire. Despite his actions, they save him and get him medical attention for his wounds. Much later, Casey speaks with Sylvie, who thanks for the rescue and tells him: “It’s always been you." This statement leaves Casey stunned. Brett starts panicking and finds Stella Kidd telling her: "I basically just told Casey that I'm in love with him." Brett and Kidd talk and Kidd asks why Casey knowing how Brett truly feels is such a problem. Brett lists her normal excuses (Dawson, work, etc.) and says that she will just have to forget about it. Casey awkwardly asks Brett if she's going out to Molly's later after shift, with her replying that she would. That night at Molly's, Brett joins the group just as Gallo starts recounting the events of the ambo accident to Capp and Tony. During their conversation, Gallo reveals that Casey jumped out of the moving Truck to get to Brett. To Brett's amazement, she realizes Casey feels the same way about her. Casey and Brett share a moment before she leaves and Gallo apologizes if he made things awkward. Casey tells him it's ok and leaves. Wanting answers, Matt shows up at Brett's apartment and asks if she has been avoiding him. She tells him that she was only avoiding him, "so I wouldn't end up doing this," and she kisses him. He then kisses her back.

Casey confessing his feelings (9x15)
Brett then admits that she'd "wanted this for a long time" and Matt agrees. Just as it seems that the two were going to get together, Brett pauses and explains the reason that she had been so scared to act on her feelings for Casey, asking him: "If Gabby came back to Chicago, right now, tonight, and asked you to leave with her, would you go?" Matt hesitates, but tells her that he hadn't spoken to Dawson in ages, but Brett doesn't see this as an answer to her question and pushes him further before he admits, "I don't know." Brett is clearly heartbroken, but thankful to Casey for his honesty. She tells Casey that what they are doing maybe isn't such a good idea. Casey tries to explain that things with Gabby were more complicated than that, but she doesn't see the situation as complicated at all. She asks Casey to leave, and although reluctant, he respects her wish. A devastated Brett then breaks down alone on her couch.

Brett's love confession (9x16)
During Smash Therapy, Casey arrives at work and spots Brett, who notices him before looking away, making him realize things are now awkward. Casey later speaks with Brett, where he tells her that he does not regret what happened between them and wants to make it work regardless of his past with Dawson. However, this explanation is not good enough for Brett and she tells him to forget that the whole thing happened. In the end, Casey spots Severide and invites him out for cigars while discussing what happened with Brett. He explains to an amused Severide that all he has been thinking about for the past year is Brett.
When Casey suffers another head injury during a call, all of his concerns about not being fit for work emerge again (after his injury in Not Like This). Brett is the first person he confides in, revealing all of his fears and uncertainties. She accompanies him during all of the medical exams. Casey later confesses that the only thing keeping him sane during this health scare was Brett.
Severide advises Casey to act on his feelings and not sit around and wait. At the end of A White-Knuckle Panic, Casey finally tells Brett that he isn't in love with Dawson anymore and that Brett is the one he's in love with.

Photo taken behind the scenes (9x16)
In No Survivors, the last episode of the season, Sylvie doesn't know how to respond to what Matt said. Her new ambo partner, Violet, tells her that Casey is a good man. Violet says that her mother's "good" describes the set of the greatest attributes a man could possibly possess. When the two respond to a man with a severe cut on his hand, his significant other tells Brett and Violet that her relationship wasn't easy but that when she knew she loved him, she knew it would always be her partner. During one shift, at night, Matt can't sleep and goes take some air outside. After a brief moment, Brett joins him outside and tells him that she also loves him restating what her patient had said earlier: "When you know, you know," and they kiss.
Later in the episode Severide and Kidd wonder why Brett and Casey aren't at Molly's Patio. It's because they are together at Brett's apartment.
Season 10[]

10x01
In Mayday, Brett and Casey are currently enjoying the "honeymoon stage" of their relationship. They try to be somewhat subtle about their new relationship status in front of their co-workers, but fail miserably. Mouch and Herrmann even discuss it while watching them leave Molly's together.
Mouch: Do they not realize we all know they're together?
Herrmann: How could we not know?!

Casey and Brett deciding to try long-distance (10x05)
Mouch: We knew a year before they did.
Harold Capp is the one to ensure Brett and Casey know about 51 knowing by calling them "lovebirds". This enables Brett and Casey to attend baby Cruz's baby shower together as a couple.
Casey fully supports Brett's Paramedicine program, which is also why they decide to try a long-distance relationship, when Casey realizes he has to move to Oregon to take care of the Darden boys, but Brett has to stay at Chicago to launch Paramedicine. (Two Hundred)
Season 11[]
In Red Waterfall at the end of the episode Matt proposes to Sylvie. The episode ended before she could answer.
Season 12[]
It is revealed in the premier that Sylvie said yes to Matt´s proposal, making them engaged. They get married and move to Portland with their family in Port in the Storm
Memorable Episodes[]
- Always - First onscreen meeting
- Try Like Hell - Olivia tells Brett she is the one meant to be with Casey
- Light Things Up - Dates to Cruz and Chloe´s wedding
- That Kind of Heat- First onscreen kiss, Casey leaps out of the firetruck still in motion
- Natural Born Firefighter - Brett acompanies Casey to his doctors visit
- A White-Knuckle Panic - Casey confesses his feelings for Brett
- No Survivors - Brett confesses her feelings for Casey, relationship began
- Two Hundred - Casey leaves for Portland, Brett stayes in Chicago
- Episodes 15-22 - Brett visits Casey in Portland
- The Magnificent City of Chicago - Dates to Severide and Kidd´s wedding
- Hold On Tight - Brett breaks up with Casey
- Danger Is All Around - Casey returns to Chicago, surprising Brett
- Red Waterfall - Casey proposes to Brett
- Barely Gone - Revealed that Brett accepted Casey´s proposal
- Port in the Storm - Brett and Casey got married
Matchmakers[]
- Foster is the first person to pick up on the tension between Brett and Casey. She is also very articulate about it, reminding Brett of her connection to Casey every time she can.
- Sylvie's friend Olivia is the one that spills the beans to Brett by telling her she and Casey are meant for each other.
- Sylvie's mom Julie was convinced Brett should act on her feelings and just forget about existence of any of the obstacles.
- Stella was very aware of Brett's attraction to Casey when she (unintentionally) pushed her to Grainger's arms. Apart from that, she usually acts in Brettsey's favor and closely observes every step they take (supported by her never-ending knowing looks).
- Severide is Casey's only confidant, therefore he knows Matt's side perfectly.
- Mouch definitely knows something's up and doesn't even try to hide it.
- Mackey was very curious about Brettsey's status and she was saddened by its platonic state (at that time).
- Violet shares her perspective with Foster and she isn't afraid to tell Brett to go for it.
- Herrmann is seemingly absolutely oblivious, because he chose Grainger as his substitute.
- Grainger was fed up with Brett's lack of interest, which eventually led him to breaking up with Sylvie and telling Matt she's in love with him.
- Capp constantly disturbs their little moments, but it seems unintentional.
- The rest of the firehouse (maybe apart from Cruz) seems to know what's going on, but they're totally indifferent.
Trivia[]
- Brett's sister, Amelia, is shown to be very familiar with Casey.
- While Casey's in Portland, he writes love letters to Brett. She is delighted by this, saying his handwriting is cute.