Sylvie joined to replace Shay. Initially Dawson was cautious when working with her but they quickly became best friends. However, when the house gets toned out for a call and Severide first sees her after returning from over a month long break, he does a double take while she's walking away from him, noticing the resemblance between her and Shay. He sees her face then boards the truck. This then escalates to the point when he tries to hang out with her, slightly using her to get past Shay since they look alike. It takes him a long time to get past this until he gets married to Brittany.
Sylvie tells the house that she ran away after getting cold feet before her marriage. However, her husband to-be finds her working at 51 and comes to visit her. He apologies in front of everyone for leaving her before their marriage, revealing that he's the one who had cold feet. Nonetheless, she gets back together with him briefly until she realizes he has a very overbearing nature. As he psychologically tries to dominate her by telling her how she should feel and where she should work. She tells him she's happy in Chicago but he tells her she isn't. She breaks up with him again and he leaves.
Sylvie joins the best rated Zumba club she found on Yelp and finds that the instructor is none other than Cruz. They agree to keep it a secret as long as she can teach him how to cook since he's opening Molly's 2, but she ends up telling Casey due to his misunderstanding of the relationship of the two. She asks if he keeps it a secret but he ends up telling it to Dawson, then Hermann, then Mouch, then everyone else. Joe then asks Sylvie out and she rejects him. However, she accepts some time later and they start dating.
She worked alongside Peter Mills when he replaced Dawson after she transferred to Truck 81 as a candidate. Mills was not cleared for duty after a head injury at the explosion where Shay died. Sylvie was upset that Mills had chosen to go to North Carolina near the end of Season 3 and during his goodbye party, she breaks down in tears and hugs Mills goodbye.
During a call, Sylvie gets attached to a baby she delivered to a gunshot victim mother who died in the ambulance. She visits the baby in the hospital several times until Chief Boden finds out and tells her to let it go and so doesChili. However, she still goes back, where she meets Dr. Charles (Chicago Med). He tells her secretly to check the father's details while the receptionist is out. She goes to see him and tells him about the son but he ignores her. However, when she comes back to see the baby yet again, the dad is there and thanks her.
She and Chili also help host a wedding, where she sings.
When Sylvie finds out Jimmy is leaving, she confesses her feelings for him, which she later tries to cover up when she finds out he is staying in the CFD and working alongside her as her new partner.
Both Chilli and Jimmy leave as paramedics, leaving Dawson to take their spot and work alongside Brett. During one of the cases, Gabby's brother Antonio shows up and Sylvie instantly develops a liking towards him. After some consideration, she asks him out but he refuses, saying that his breakup with his ex-wife is still on his mind and he doesn't want her mixed up with this. She responds by saying that she doesn't mind and as a result, they start going out.
Sylvie tries harder to impress Antonio's son Diego, who likes to play chess and whom she doesn't connect with. Otis teaches her and Antonio asks her later to take care of Diego when he's suddenly called to work. She agrees and her and Diego are at the firehouse when Antonio's ex-wife, Laura, shows up abruptly and asks Diego to wait for her while she harshly tells Sylvie that she doesn't trust her and doesn't want a girl like her around her son. This upsets Sylvie and when Antonio shows up, she tells him what happened and he apologies and she adds that she doesn't want to be in the middle of his fight with him and his ex-wife. He tells her that he warned her before this but she replies that she didn't expect to be directly confronted by Laura. He lashes out and she breaks up with him.
Antonio later shows up at Sylvie's door and apologies but she is still angry. She later (5x12) tries to get her mind off Antonio by doing extra exercise at the firehouse, trying to find online makeup classes and changing her look and even learning German. Dawson feels bad and tries setting her up with people Casey knows but Casey doesn't want to get involved in her love-life.