Hallie and Casey were engaged, but Hallie broke it off. They later reconciled when Casey asked if they could start fresh.
Later, Hallie informs Casey that she doesn't want to have children. Because of this, they break up again and she moves to South America. However, she returns later, near the end of Season 1, and they reconcile again.
In reference to episode "Leaders Lead".
Hallie was murdered, when she uncovered and witnessed a drug operation going on in the clinic that she worked in. She was murdered, and then the clinic was set on fire.
Casey found her once they got a call to the clinic once it was set on fire, and she was rushed to the hospital. However, it was too late, and she later died, devastating Casey.
During the course of the subsequent investigation, Antonio Dawson tells Casey that Hallie was murdered when she was struck on the back of her head prior to the fire. It was later revealed that the person responsible was Timothy Campbell, who was later fatally shot by Voight.