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).
Cassidy had her breakthrough in 2012, after being cast as Laurel Lance Black Siren, an antagonistic version of the character from a parallel universe, originating on The Flash and continuing on to Arrow.