Hallie Thomas was a doctor and one of the characters of Chicago Fire.
Hallie was engaged to Matthew Casey. But left in order to find herself, she soon returned in episode 22 to reconcile. she later died in a fire and her death left Casey heartbroken.
It was discovered that Hallie was murdered before the clinic caught fire and the person who killed her is Timothy Campbell.
(To:Casey) "That sneaky little bastard" 1x21