Katherine Evelyn Anita Cassidy (born November 25, 1986) 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 mystery horror 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 Canary, on The CW television series Arrow. Cassidy has reprised her role in the Arrowverse series The Flash and Legends of Tomorrow, as well as voicing the character in the animated web series Vixen. Following her character's death in season four of Arrow, Cassidy began playing Laurel Lance / Black Siren, an antagonistic version of the character from a parallel universe, originating on The Flash and continuing on to Arrow.