Heather Darden was the wife of the now deceased Andrew Darden, killed in the line of duty in the Pilot episode. She and Andy had two kids, both boys, called Griffin and Ben. She remained close friends with Matthew Casey after her husbands death, and even made sexual advances onto him, which she shut down as it wasn't right, because of Casey and Darden's friendship. She blamed Kelly Severide for her husband's death as she believed that he was the reason that Andy wanted to become a firefighter.

Initially, she doesn't talk to Severide and avoids him, only talking to Casey. This bothers him as he doesn't understand why she's so close to him. He even tries to talk to her and helps her with a few chores but she's resistant and eventually outright tells him that she blames him for her husband's death. This worsens when he comes over to see Casey, and finds her sleeping over at his house, making him think that they're going out. Kelly was angry at Casey for a few episodes before everything was cleared up, as they realised it was stupid to make accusations like that. Heather begins to act civil with Severide after she realises her husbands death was not his fault. 

Drunk Driving Incident

In reference to episode "A Problem House".

On the 1 year anniversary of Andy's death, Heather goes out drinking with some friends. Meanwhile, Firehouse 51 is called to a motor vehicle accident, and Casey recognises the car immediately - and sees Heather standing across the street, talking to the police. Since she'd been drinking, she tested positive on the breathalizer. Heather's friend in the car, in the passenger seat, was killed from her injuries, and so Heather pleads guilty to DUI Manslaughter, and is sentenced with 15 months. She asks Casey to take care of Ben and Griffin, who are reluctant to see their mother go to prison. However, near the end of Season 1 her sentence is reduce, and at the beginning of Season 2 moves away from Chicago with Ben and Griffin.