A pre-teen during Season 1, and her parents break-up, Cassidy grew up developing issues against her parents. As she became a teen, she lashed out constantly at both Vic and Corrine. At the end of the show, she seemed to have settled down and took care of her younger siblings.
Cassidy got angry at his brother Matthew because he ruined his notebook. Vic then took her aside and tried to explain how Matthew saw things different and that sometimes he might need special "big sister" help. That night, as the family was eating pizza, Matthew was struggling to eat it with a spoon. Vic tried to help him unsuccessfully, but Cassidy took her own slice and showed Matthew how it was done. Matthew then did the same.
Some time later, Ben Gilroy visited Mackey's house while he wasn't there under the pretext that Vic had invited him. As he talked to Mackey on the phone, threatening him for investigating him, Cassidy told Ben she wanted to show him some drawings. Gilroy hung up on Vic and went to her room with her. While there, he hid a loaded gun he had used to kill a witness in an air vent. When Mackey arrived looking for Ben, Corrine told him he had left. But Vic's sudden arrival scared Cassidy. He then took her in his arms and told Corrine to pack their belongings. He then took them to a hotel and had Lem take care of them, until he captured Gilroy.
The next day, Corrine returned to her home only to find it destroyed. She then decided to take the kids and leave Vic.