Cassie Robinson is the daughter of Martin and Audrey Robinson. She was in a long term relationship with Dean before he joined Sam in the hunting life. They separated due to her inability to cope with Dean's life as a hunter.
Cassie got a job as a reporter and eventually met Dean at some unknown point. Dean and Cassie dated and eventually became serious enough that Dean explained "his job" to her but she did not believe him and thought that it was a lie made up to cover for his disappearances.
After Cassie's father, a car dealership owner, was killed by the spirit of Cyrus Dorian, Cassie contacted Dean to help her investigate, finally accepting the truth about the supernatural. Despite this, once seeing him again, uneasiness between them remains evident.
After the death of her father's best friend, Jimmy, Cassie notices a seemingly non-chalant attitude towards the Mayor regarding the subject and accuses him of being racist, but when she is told that she of all people should know that he is not, she becomes suspicious about the meaning behind that.
Later on, her tensions with Dean and their failed relationship reaches a boiling point, leading them to have passionate sex. She and Dean speak to each other regarding no more secrets between them and question if their relationship can progress after this.
Sometime afterwards, as Cassie is looking more into the death of her father, his friends, and most recently the mayor, she is antagonized by the spirit of Cyrus Dorian, who wishes to kill her mother and her next. After learning the truth about her parents' history with Cyrus Dorian, she aids Dean and Sam in ending the threat.
Afterwards, Cassie and Dean finally realize that their relationship cannot blossom due to Dean's life as a hunter and regretfully depart with a kiss.
Cassie's romance with Dean and Cyrus Dorian' rampage are among the events recorded in Chuck Shurley's Supernatural book series revolved around Sam and Dean's lives, with these events specifically appearing as the plot of Route 666. When Dean discovers and reads the books he's highly disturbed that his romance with Cassie is written in vivid detail, describing it as "full frontal".