Katherine Evelyn Anita Cassidy is an American actress. Following several minor television roles, she came to attention as a scream queen after starring in the horror films When a Stranger Calls (2006), as Kelli Presley in Black Christmas (2006) and as Ruby in the third season of the horror series Supernatural (2007–2008). Following a supporting role in the action film Taken (2008), Cassidy played leading roles in the slasher series Harper's Island (2009) and the remake of the drama series Melrose Place (2009–2010). She starred as Kris Fowles in the slasher film remake A Nightmare on Elm Street (2010) and had a recurring role as Juliet Sharp during the fourth season of the teen drama Gossip Girl (2010–2012).

Ruby is a demon on The CW Television Network's Supernatural portrayed mainly by actresses Katie Cassidy and Genevieve Cortese. Created by the writers to expand on the characterization of demons within the series, she first appears in the third season, wherein she assists series protagonists Sam and Dean Winchester in fighting her fellow demons. By the fourth season, she has won Sam's trust and begins training him to kill demons with his psychic powers, though Dean remains fearful of ulterior motives. The character is killed at the end of the fourth season. In the fifteenth season, Ruby returns through flashbacks and a visit to the Empty, the angels' and demons' afterlife.