Supernatural did not see favored cast member Katie Cassidy return after season 3, and the reason why could be one of two explanations. In the third season of the long-running sci-fi series, Cassidy originated the character of Ruby, a demon who tried to tear the Winchester brothers apart while pretending to be an ally. Ruby was a fan favorite, but she was one of many characters who did not stick around for the entirety of Supernatural's existence — a decision that eventually hurt the show.
The CW drama revolved around two brothers, Dean and Sam Winchester, who battled different monsters from episode to episode in a thrilling blend of intriguing performances, humor, and witty scriptwriting. In Supernatural season 3, episode 1, “The Magnificent Seven,” Ruby possessed an unknown blonde woman and followed Sam, eventually defending him from other demons by killing them with a special knife. An alliance between Ruby and Sam then formed when she promised to help save his brother. Dean had made a deal with a demon to resurrect Sam, and for most of season 3, the brothers looked for ways to break away from the deal, but in the end, Dean died and went to Hell.
Katie Cassidy was the first of two actors to play Ruby in Supernatural, and while she admitted to loving her time with the cast, she still decided to leave. The show's creators cited budget limitations as the reason for the actress's short-lived stint as Ruby, claiming that they could no longer afford to pay her (via TV Guide). But Cassidy has alleged that at the time, the writers did not seem to have an idea of the character’s direction, and when she got an opportunity to star in the limited series Harper’s Island, she took it, voluntarily quitting Supernatural in 2008. From there, Cassidy's career skyrocketed, with major roles in movies like the 2010 A Nightmare on Elm Street remake and in The CW's Arrowverse shows, where she portrayed DC superhero Black Canary.
After Katie Cassidy’s exit from Supernatural, Genevieve Cortese took over the role. In season 4, after taking on a new form, Ruby regained Sam’s trust and trained him to kill demons with his psychic powers, but Dean remained wary, and understandably so. The initial reception toward Cortese taking over the role of Ruby after season 3 was largely negative, with critics describing her performance as underwhelming. However, as the show progressed, the audience became more accepting of the actor’s portrayal of the demon character.
Ruby eventually died in the season 4 finale, "Lucifer Rising," but that wasn't the end of Cortese's involvement with the show. She not only went on to marry Supernatural star Jared Padalecki in real-life (changing her stage name to Genevieve Padalecki) but she also returned years after playing Ruby to guest-star as a fictional version of herself in Supernatural season 6, episode 15, “The French Mistake.”