10. Hallie Thomas and Matt Casey
Hallie Thomas (Teri Reeves) and Matt Casey (Jesse Spencer) had the rare Chicago Fire romance that predated the show. As we learn in season 1, the couple have been on and off for a whopping 8 years, which didn't instill a ton of confidence in their future.
It was sad to see Thomas get killed in the season 1 finale, but her time with Casey felt so flat that it was more of an instance of feeling bad for Casey than it was concern over the fact that we would never get to see the character again. Things improved drastically from here, relationship wise.