Hallie Thomas was a doctor and one of the characters of Chicago Fire.
Hallie had an on-and-off relationship with Matthew Casey, they were together for 8 years and were engaged before she broke it off. They later reconciled when Casey asked whether they could start fresh, which she agrees to. Hallie later informs Casey of the fact that she doesn't want to have children and only said she did because he wants to start a family with her. Because of this, they break up again and she moved to South America. She comes back near the end of season 1 and her and Casey reconcile and start up their relationship again, however she is later killed which leaves Casey heartbroken.
She was engaged and had a relationship with Matthew Casey for 8 years, during this time they broke up and then got back together. She was caught in a fire at the clinic that she worked at and was rescued by Casey and Severide, however she later died at the hospital. When her death is being investigated, Antonio Dawson tells Casey that Hallie was murdered by being hit on the back of the head before the clinic caught fire. It was later revealed that the person who killed her is Timothy Campbell, who was later shot in the head by Voight.
(To Casey about Mills and Dawson) "That sneaky little bastard" 1x21